Using vim as a mergetool in git

Setup:

git config --global merge.tool gvimdiff
git config --global mergetool prompt false
git config --global merge.conflictstyle diff3

The last is so that we can see the base revision displayed within the merge markers, in the merged file. The base the revision that was the most recent common basis for the two different versions implied in the merge.

Rebasing:

git rebase main-master
git mergetool

Quoting from git mergetool --help:

When git mergetool is invoked with this tool (either through the -t or –tool option or the merge.tool configuration variable) the configured command line will be invoked with $BASE set to the name of a temporary file containing the common base for the merge, if available; $LOCAL set to the name of a temporary file containing the contents of the file on the current branch; $REMOTE set to the name of a temporary file containing the contents of the file to be merged, and $MERGED set to the name of the file to which the merge tool should write the result of the merge resolution.

Interpreting the text above in the context of git rebase --help - the “current branch” will, in this case, be main-master - more generally it will be the “upstream” branch in the rebase command line, or “newbase” if you used the --onto <newbase> option.

On my screen, there are four windows, three at the top, and one below. (This is for git 1.7.4.1. Versions at least until 1.7.2 had three windows, vertically arranged, being LOCAL, MERGED and REMOTE in the terms below). In a rebase, the ones at the top will be

  • LOCAL : the current branch (git branch). In the case of a rebase this will be the upstream branch (or <newbase> from --onto). Here it would be main-master.

  • BASE : the common basis against which LOCAL and REMOTE have changed. This allows us to see what LOCAL and REMOTE have changed against.

  • REMOTE : the file as for the changes we are adding with the rebase. In our case, that would be the branch we were on when we issued git rebase main-master.

The one at the bottom is:

  • MERGED : the file to which we write the changes, to be selected from LOCAL (new changes) REMOTE (upstream changes). This is the file with the merge markers in it. If you have conflictstyle above set to diff3 you will see the LOCAL, REMOTE and BASE versions within the mergemarker block.

Now, time to go through the lower pane, pulling in changes from the “upstream” (left) pane, or the “rebase changes” (right) pane, as in one of:

:diffg LO
:diffg BA
:diffg RE