Git merge in depth

How exactly 3-way recursive merge and rebase work.

Transcript

1. git merge in depth
   Anton Chikin
   

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2. test
   branch
   merge
   master
   to branch
   test
   branch
   merge
   branch
   to master
   test
   master
   

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3. test
   branch
   merge
   master
   to branch
   test
   branch
   merge
   branch
   to master
   test
   master
   WAT
   

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4. I don’t trust merge!
   

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7. 3-way merge
   

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8. master
   

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9. master
   branch
   

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10. master
    branch
    

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11. master
    branch
    

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12. master
    branch
    

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13. master
    branch
    

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14. master
    branch
    ?
    

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15. master
    branch
    ?
    var a = “foo”
    def getData(string
    path)
    var data = Map()
    for l in
    visit(path)
    data[l.slot] =
    l.data
    return
    data.JSON()
    var a = “foo”
    def getData(URI
    path)
    var data =
    Array()
    for l in
    visit(path)
    data[l.id] =
    l.data
    return data
    

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16. ?
    var a = “foo”
    def getData(string path)
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    var a = “foo”
    def getData(URI path)
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    master
    branch
    

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17. ?
    var a = “foo”
    def getData(string path)
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    var a = “foo”
    def getData(URI path)
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    master
    branch
    ?
    

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18. ?
    var a = “foo”
    def getData(string path)
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    var a = “foo”
    def getData(URI path)
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    master
    branch
    ?
    ?
    

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19. ?
    var a = “foo”
    def getData(string path)
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    var a = “foo”
    def getData(URI path)
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    master
    branch
    ?
    ?
    ?
    

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20. ?
    var a = “foo”
    def getData(string path)
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    var a = “foo”
    def getData(URI path)
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    master
    branch
    ?
    ?
    ?
    ?
    

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21. ?
    var a = “foo”
    def getData(string path)
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    var a = “foo”
    def getData(URI path)
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    master
    branch
    ?
    ?
    ?
    ?
    Nearest common ancestor
    var a = “foo”
    def getData(string
    path)
    var data = {}
    for l in
    visit(path)
    data[l.slot] =
    l.data
    return data
    

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22. ?
    var a = “foo”
    def getData(string path)
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    var a = “foo”
    def getData(URI path)
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    var a = “foo”
    def getData(string path)
    var data = {}
    for l in visit(path)
    data[l.slot] = l.data
    return data
    ancestor
    master
    branch
    

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23. ?
    var a = “foo”
    def getData(string path)
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    var a = “foo”
    def getData(URI path)
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    var a = “foo”
    def getData(string path)
    var data = {}
    for l in visit(path)
    data[l.slot] = l.data
    return data
    ancestor
    merge
    ancestor
    master
    branch
    

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24. ?
    var a = “foo”
    def getData(string path)
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    var a = “foo”
    def getData(URI path)
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    var a = “foo”
    def getData(string path)
    var data = {}
    for l in visit(path)
    data[l.slot] = l.data
    return data
    ancestor
    merge
    ancestor
    master
    branch
    def getData(string path)
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    def getData(string path)
    var data = {}
    for l in visit(path)
    data[l.slot] = l.data
    return data
    def getData(URI path)
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    var a = “foo”
    var a = “foo”
    var a = “foo”
    var a = “foo”
    

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25. ?
    var a = “foo”
    def getData(string path)
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    var a = “foo”
    def getData(URI path)
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    var a = “foo”
    def getData(string path)
    var data = {}
    for l in visit(path)
    data[l.slot] = l.data
    return data
    ancestor
    merge
    ancestor
    master
    branch
    def getData(string path)
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    def getData(string path)
    var data = {}
    for l in visit(path)
    data[l.slot] = l.data
    return data
    def getData(URI path)
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    var a = “foo”
    var a = “foo”
    var a = “foo”
    var a = “foo”
    

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26. ?
    var a = “foo”
    def getData(string path)
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    var a = “foo”
    def getData(URI path)
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    var a = “foo”
    def getData(string path)
    var data = {}
    for l in visit(path)
    data[l.slot] = l.data
    return data
    ancestor
    merge
    ancestor
    master
    branch
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    var data = {}
    for l in visit(path)
    data[l.slot] = l.data
    return data
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    var a = “foo”
    var a = “foo”
    var a = “foo”
    var a = “foo”
    def getData(string path) def getData(URI path)
    def getData(string path)
    

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27. ?
    var a = “foo”
    def getData(string path)
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    var a = “foo”
    def getData(URI path)
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    var a = “foo”
    def getData(string path)
    var data = {}
    for l in visit(path)
    data[l.slot] = l.data
    return data
    ancestor
    merge
    ancestor
    master
    branch
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    var data = {}
    for l in visit(path)
    data[l.slot] = l.data
    return data
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    var a = “foo”
    var a = “foo”
    var a = “foo”
    var a = “foo”
    def getData(string path)
    def getData(URI path)
    def getData(string path)
    

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28. def getData(string path)
    ?
    var a = “foo”
    def getData(string path)
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    var a = “foo”
    def getData(URI path)
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    var a = “foo”
    def getData(string path)
    var data = {}
    for l in visit(path)
    data[l.slot] = l.data
    return data
    ancestor
    merge
    ancestor
    master
    branch
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    for l in visit(path)
    data[l.slot] = l.data
    return data
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    var a = “foo”
    var a = “foo”
    var a = “foo”
    var a = “foo”
    def getData(string path)
    def getData(URI path)
    var data = {}
    

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29. def getData(string path)
    ?
    var a = “foo”
    def getData(string path)
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    var a = “foo”
    def getData(URI path)
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    var a = “foo”
    def getData(string path)
    var data = {}
    for l in visit(path)
    data[l.slot] = l.data
    return data
    ancestor
    merge
    ancestor
    master
    branch
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    for l in visit(path)
    data[l.slot] = l.data
    return data
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    var a = “foo”
    var a = “foo”
    var a = “foo”
    var a = “foo”
    def getData(string path)
    def getData(URI path)
    var data = {}
    ?????????????
    

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30. def getData(string path)
    ?
    var a = “foo”
    def getData(string path)
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    var a = “foo”
    def getData(URI path)
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    var a = “foo”
    def getData(string path)
    var data = {}
    for l in visit(path)
    data[l.slot] = l.data
    return data
    ancestor
    merge
    ancestor
    master
    branch
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    for l in visit(path)
    data[l.slot] = l.data
    return data
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    var a = “foo”
    var a = “foo”
    var a = “foo”
    var a = “foo”
    def getData(string path)
    def getData(URI path)
    var data = {}
    ?????????????
    

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31. def getData(string path)
    <<<<<<< HEAD
    var data = Map()
    ||||||| merged common ancestors
    var data = {}
    =======
    var data = Array()
    >>>>>>> branch
    

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32. def getData(string path)
    ?
    var a = “foo”
    def getData(string path)
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    var a = “foo”
    def getData(URI path)
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    var a = “foo”
    def getData(string path)
    var data = {}
    for l in visit(path)
    data[l.slot] = l.data
    return data
    ancestor
    merge
    ancestor
    master
    branch
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    for l in visit(path)
    data[l.slot] = l.data
    return data
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    var a = “foo”
    var a = “foo”
    var a = “foo”
    var a = “foo”
    def getData(string path)
    def getData(URI path)
    var data = {}
    var data = Map()
    for l in visit(path)
    

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33. def getData(string path)
    ?
    var a = “foo”
    def getData(string path)
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    var a = “foo”
    def getData(URI path)
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    var a = “foo”
    def getData(string path)
    var data = {}
    for l in visit(path)
    data[l.slot] = l.data
    return data
    ancestor
    merge
    ancestor
    master
    branch
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    for l in visit(path)
    data[l.slot] = l.data
    return data
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    var a = “foo”
    var a = “foo”
    var a = “foo”
    var a = “foo”
    def getData(string path)
    def getData(URI path)
    var data = {}
    var data = Map()
    for l in visit(path)
    

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34. def getData(string path)
    ?
    var a = “foo”
    def getData(string path)
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    var a = “foo”
    def getData(URI path)
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    var a = “foo”
    def getData(string path)
    var data = {}
    for l in visit(path)
    data[l.slot] = l.data
    return data
    ancestor
    merge
    ancestor
    master
    branch
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    for l in visit(path)
    data[l.slot] = l.data
    return data
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    var a = “foo”
    var a = “foo”
    var a = “foo”
    var a = “foo”
    def getData(string path)
    def getData(URI path)
    var data = {}
    var data = Map()
    for l in visit(path)
    data[l.id] = l.data
    

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35. def getData(string path)
    ?
    var a = “foo”
    def getData(string path)
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    var a = “foo”
    def getData(URI path)
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    var a = “foo”
    def getData(string path)
    var data = {}
    for l in visit(path)
    data[l.slot] = l.data
    return data
    ancestor
    merge
    ancestor
    master
    branch
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    for l in visit(path)
    data[l.slot] = l.data
    return data
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    var a = “foo”
    var a = “foo”
    var a = “foo”
    var a = “foo”
    def getData(string path)
    def getData(URI path)
    var data = {}
    var data = Map()
    for l in visit(path)
    data[l.id] = l.data
    

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36. def getData(string path)
    ?
    var a = “foo”
    def getData(string path)
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    var a = “foo”
    def getData(URI path)
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    var a = “foo”
    def getData(string path)
    var data = {}
    for l in visit(path)
    data[l.slot] = l.data
    return data
    ancestor
    merge
    ancestor
    master
    branch
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    for l in visit(path)
    data[l.slot] = l.data
    return data
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    var a = “foo”
    var a = “foo”
    var a = “foo”
    var a = “foo”
    def getData(string path)
    def getData(URI path)
    var data = {}
    var data = Map()
    for l in visit(path)
    data[l.id] = l.data
    return data.JSON()
    

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37. def getData(string path)
    ?
    var a = “foo”
    def getData(string path)
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    var a = “foo”
    def getData(URI path)
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    var a = “foo”
    def getData(string path)
    var data = {}
    for l in visit(path)
    data[l.slot] = l.data
    return data
    ancestor
    merge
    ancestor
    master
    branch
    var data = Map()
    for l in visit(path)
    data[l.slot] = l.data
    return data.JSON()
    for l in visit(path)
    data[l.slot] = l.data
    return data
    var data = Array()
    for l in visit(path)
    data[l.id] = l.data
    return data
    var a = “foo”
    var a = “foo”
    var a = “foo”
    var a = “foo”
    def getData(string path)
    def getData(URI path)
    var data = {}
    var data = Map()
    for l in visit(path)
    data[l.id] = l.data
    return data.JSON()
    

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38. master
    branch
    ?
    

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39. master
    branch
    

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40. master
    branch
    merge master into branch
    

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41. master
    branch
    merge master into branch
    

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42. branch
    merge branch into master
    master
    

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43. recursive 3-way merge
    

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44. master
    Let’s take a look at a more complicated picture
    

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45. master
    hotfix
    Let’s take a look at a more complicated picture
    

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46. master
    hotfix
    Let’s take a look at a more complicated picture
    

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47. master
    feature
    hotfix
    Let’s take a look at a more complicated picture
    

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48. master
    hotfix
    feature
    Let’s take a look at a more complicated picture
    

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49. master
    hotfix
    feature
    Let’s take a look at a more complicated picture
    

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50. master
    M
    hotfix
    feature
    Let’s take a look at a more complicated picture
    

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51. master
    M
    M
    hotfix
    feature
    Let’s take a look at a more complicated picture
    

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52. master
    M
    M
    ?
    hotfix
    feature
    Let’s take a look at a more complicated picture
    

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53. master
    M
    M
    ?
    hotfix
    feature
    Let’s take a look at a more complicated picture
    nearest common ancestor
    

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54. master
    M
    M
    ?
    hotfix
    feature
    How to deal with two nearest common ancestors?
    nearest common ancestor
    nearest common ancestor
    

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55. master
    M
    M
    ?
    hotfix
    nearest common ancestor
    Step 1: find nearest common ancestor
    of those ancestors
    

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56. master
    M
    M
    ?
    hotfix
    Step 2: merge them into “fake” commit
    nearest common ancestor
    

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57. master
    M
    M
    ?
    hotfix
    Step 3: Merge our commits using that “fake” ancestor
    nearest common ancestor
    

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58. master
    M
    M
    hotfix
    M
    nearest common ancestor
    This process can be recursive
    

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59. master
    M
    M
    hotfix
    feature
    M
    

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60. And what about rebase?
    

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61. master
    branch
    master
    A B C
    

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62. master
    branch
    master
    A B C
    nearest common ancestor
    Again, find NCO
    

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63. master
    branch
    master
    diff B diff C
    diff A
    A B C
    Create diffs between each commits starting from NCO
    

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64. master
    branch
    master
    A B C
    diff B diff C
    diff A
    Move branch head to master
    

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65. master
    branch
    master
    A B C
    A’
    diff B diff C
    Apply each diff on top of latest master commit
    

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66. master
    branch
    master
    A B C
    A’ B’
    diff C
    Apply each diff on top of latest master commit
    

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67. master
    branch
    master
    A B C
    A’ B’ C’
    Apply each diff on top of latest master commit
    

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68. @1,1
    - A
    + B
    What does it mean to “apply diff”?
    Diff format is quite simple:
    In the first line
    

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69. @1,1
    - A
    + B
    Diff format is quite simple:
    In the first line
    replace “A”
    What does it mean to “apply diff”?
    

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70. @1,1
    - A
    + B
    Diff format is quite simple:
    In the first line
    replace “A”
    with “B”
    What does it mean to “apply diff”?
    

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71. A
    B
    C
    @1,1
    - A
    + B
    B
    B
    C
    What does it mean to “apply diff”?
    

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72. D
    B
    C
    @1,1
    - A
    + B
    ????????
    B
    C
    What if first line in file is not “A” but “D”?
    Same as in merge - conflict!
    What does it mean to “apply diff”?
    

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73. D
    B
    C
    @1,1
    - A
    + B
    ????????
    B
    C
    What if first line in file is not “A” but “D”?
    Same as in merge - conflict!
    What does it mean to “apply diff”?
    

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74. master
    branch
    branch
    master
    A’ B’ C’
    A B C
    M
    Merge and rebase give you different branch structure
    but the same result
    

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75. Merge commits “train station”
    

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76. Merge commits “subway”
    

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77. Finally
    - don’t be afraid of recursive merge
    - merge a ->b == merge b ->a
    - merge == rebase
    - prefer merge over rebase
    - merge will never never let you down
    

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78. •http://think-like-a-git.net/
    •http://git-school.github.io/visualizing-git/
    Resources
    

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79. Questions?
    

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80. git merge in depth
    Anton Chikin
    

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