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   1 From: Junio C Hamano <gitster@pobox.com>
   2 Subject: Separating topic branches
   3 Abstract: In this article, JC describes how to separate topic branches.
   4
   5 This text was originally a footnote to a discussion about the
   6 behaviour of the git diff commands.
   7
   8 Often I find myself doing that [running diff against something other
   9 than HEAD] while rewriting messy development history.  For example, I
  10 start doing some work without knowing exactly where it leads, and end
  11 up with a history like this:
  12
  13             "master"
  14         o---o
  15              \                    "topic"
  16               o---o---o---o---o---o
  17
  18 At this point, "topic" contains something I know I want, but it
  19 contains two concepts that turned out to be completely independent.
  20 And often, one topic component is larger than the other.  It may
  21 contain more than two topics.
  22
  23 In order to rewrite this mess to be more manageable, I would first do
  24 "diff master..topic", to extract the changes into a single patch, start
  25 picking pieces from it to get logically self-contained units, and
  26 start building on top of "master":
  27
  28         $ git diff master..topic >P.diff
  29         $ git checkout -b topicA master
  30         ... pick and apply pieces from P.diff to build
  31         ... commits on topicA branch.
  32
  33               o---o---o
  34              /        "topicA"
  35         o---o"master"
  36              \                    "topic"
  37               o---o---o---o---o---o
  38
  39 Before doing each commit on "topicA" HEAD, I run "diff HEAD"
  40 before update-index the affected paths, or "diff --cached HEAD"
  41 after.  Also I would run "diff --cached master" to make sure
  42 that the changes are only the ones related to "topicA".  Usually
  43 I do this for smaller topics first.
  44
  45 After that, I'd do the remainder of the original "topic", but
  46 for that, I do not start from the patchfile I extracted by
  47 comparing "master" and "topic" I used initially.  Still on
  48 "topicA", I extract "diff topic", and use it to rebuild the
  49 other topic:
  50
  51         $ git diff -R topic >P.diff ;# --cached also would work fine
  52         $ git checkout -b topicB master
  53         ... pick and apply pieces from P.diff to build
  54         ... commits on topicB branch.
  55
  56                                 "topicB"
  57                o---o---o---o---o
  58               /
  59              /o---o---o
  60             |/        "topicA"
  61         o---o"master"
  62              \                    "topic"
  63               o---o---o---o---o---o
  64
  65 After I am done, I'd try a pretend-merge between "topicA" and
  66 "topicB" in order to make sure I have not missed anything:
  67
  68         $ git pull . topicA ;# merge it into current "topicB"
  69         $ git diff topic
  70                                 "topicB"
  71                o---o---o---o---o---* (pretend merge)
  72               /                   /
  73              /o---o---o----------'
  74             |/        "topicA"
  75         o---o"master"
  76              \                    "topic"
  77               o---o---o---o---o---o
  78
  79 The last diff better not to show anything other than cleanups
  80 for crufts.  Then I can finally clean things up:
  81
  82         $ git branch -D topic
  83         $ git reset --hard HEAD^ ;# nuke pretend merge
  84
  85                                 "topicB"
  86                o---o---o---o---o
  87               /
  88              /o---o---o
  89             |/        "topicA"
  90         o---o"master"