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git-imerge -- incremental merge and rebase for git

Perform the merge between two branches incrementally. If conflicts are encountered, figure out exactly which pairs of commits conflict, and present the user with one pairwise conflict at a time for resolution.

git-imerge has two primary design goals:

  • Reduce the pain of resolving merge conflicts to its unavoidable minimum, by finding and presenting the smallest possible conflicts: those between the changes introduced by one commit from each branch.
  • Allow a merge to be saved, tested, interrupted, published, and collaborated on while it is in progress.

I think that it is easiest to understand the concept of incremental merging visually, and therefore I recommend the video of my git-imerge presentation from the GitMerge 2013 conference (20 min) as a good place to start. The full slides for that talk are available in this repository under doc/presentations/GitMerge-2013. At the same conference, I was interviewed about git-imerge by Thomas Ferris Nicolaisen for his GitMinutes Podcast #12.

To learn how to use the git-imerge tool itself, I suggest the blog article git-imerge: A Practical Introduction and also typing git-imerge --help and git-imerge SUBCOMMAND --help. If you want more information, the theory and benefits of incremental merging are described in minute detail in a series of blog articles [1], as are the benefits of retaining history when doing a rebase [2].

Multiple incremental merges can be in progress at the same time. Each incremental merge has a name, and its progress is recorded in the Git repository as references under refs/imerge/NAME. The current state of an incremental merge can be visualized using the diagram command.

An incremental merge can be interrupted and resumed arbitrarily, or even pushed to a server to allow somebody else to work on it.

git-imerge is experimental! If it breaks, you get to keep the pieces. For example, it is recommended that you make a clone of your git repository and run the script on the clone rather than the original. Feedback and bug reports are welcome!

Instructions

For basic operation, you only need to know three git-imerge commands. To merge BRANCH2 into BRANCH1 or rebase BRANCH2 onto BRANCH1,

git checkout BRANCH1
git-imerge start --name=NAME --goal=GOAL --first-parent BRANCH2
while not done:
    <fix conflict presented to you and "git add" the changes>
    git-imerge continue
git-imerge finish

where

NAME
is the name for this merge (and also the default name of the branch to which the results will be saved).
GOAL describes how you want to simplify the results (see next
section).

Simplifying results

When the incremental merge is finished, you can simplify its results in various ways before recording it in your project's permanent history.

discard the intermediate merge commits and create a simpler history to record permanently in your project repository using either the finish or simplify command. The incremental merge can be simplified in one of four ways:

merge

keep only a simple merge of the second branch into the first branch, discarding all intermediate merges. The end result is similar to what you would get from

git checkout BRANCH1
git merge BRANCH2
manual
like merge, but also keep any merges that had to be done manually plus enough automatic merges to connect the manual merges together meaningfully. If the result is later discovered to be buggy, the additional information might be helpful in isolating the problem.
rebase

keep the versions of the commits from the second branch rebased onto the first branch. The end result is similar to what you would get from

git checkout BRANCH2
git rebase BRANCH1
rebase-with-history

like rebase, except that it retains the old versions of the rebased commits in the history. It is equivalent to merging the commits from BRANCH2 into BRANCH1, one commit at a time. In other words, it transforms this:

o---o---o---o          BRANCH1
     \
      A---B---C---D    BRANCH2

into this:

o---o---o---o---A'--B'--C'--D'   NEW_BRANCH
     \         /   /   /   /
      --------A---B---C---D

It is safe to rebase an already-published branch using this approach. See [2] for more information.

full
don't simplify the incremental merge at all: do all of the intermediate merges and retain them all in the permanent history.

License

git-imerge is released as open-source software under the GNU General Public License (GPL), version 2 or later.

References

[1]
[2](1, 2)