How to separate topic branches

This text was originally a footnote to a discussion about the behaviour of the git diff commands.

Often I find myself doing that [running diff against something other than HEAD] while rewriting messy development history. For example, I start doing some work without knowing exactly where it leads, and end up with a history like this:

    "master"
o---o
     \                    "topic"
      o---o---o---o---o---o

At this point, "topic" contains something I know I want, but it contains two concepts that turned out to be completely independent. And often, one topic component is larger than the other. It may contain more than two topics.

In order to rewrite this mess to be more manageable, I would first do "diff master..topic", to extract the changes into a single patch, start picking pieces from it to get logically self-contained units, and start building on top of "master":

$ git diff master..topic >P.diff
$ git checkout -b topicA master
... pick and apply pieces from P.diff to build
... commits on topicA branch.

      o---o---o
     /        "topicA"
o---o"master"
     \                    "topic"
      o---o---o---o---o---o

Before doing each commit on "topicA" HEAD, I run "diff HEAD" before update-index the affected paths, or "diff --cached HEAD" after. Also I would run "diff --cached master" to make sure that the changes are only the ones related to "topicA". Usually I do this for smaller topics first.

After that, I’d do the remainder of the original "topic", but for that, I do not start from the patchfile I extracted by comparing "master" and "topic" I used initially. Still on "topicA", I extract "diff topic", and use it to rebuild the other topic:

$ git diff -R topic >P.diff ;# --cached also would work fine
$ git checkout -b topicB master
... pick and apply pieces from P.diff to build
... commits on topicB branch.

                        "topicB"
       o---o---o---o---o
      /
     /o---o---o
    |/        "topicA"
o---o"master"
     \                    "topic"
      o---o---o---o---o---o

After I am done, I’d try a pretend-merge between "topicA" and "topicB" in order to make sure I have not missed anything:

$ git pull . topicA ;# merge it into current "topicB"
$ git diff topic
                        "topicB"
       o---o---o---o---o---* (pretend merge)
      /                   /
     /o---o---o----------'
    |/        "topicA"
o---o"master"
     \                    "topic"
      o---o---o---o---o---o

The last diff better not to show anything other than cleanups for cruft. Then I can finally clean things up:

$ git branch -D topic
$ git reset --hard HEAD^ ;# nuke pretend merge

                        "topicB"
       o---o---o---o---o
      /
     /o---o---o
    |/        "topicA"
o---o"master"