Merge branch 'sb/merge-commit-msg-hook'

As "git commit" to conclude a conflicted "git merge" honors the
commit-msg hook, "git merge" that recoreds a merge commit that
cleanly auto-merges should, but it didn't.

* sb/merge-commit-msg-hook:
builtin/merge: honor commit-msg hook for merges

diff --cc builtin/merge.c
Simple merge