This fixes a crash in an interactive rebase when there's a merge command in the
rebase-todo file that doesn't have a comment. I don't know under what
circumstances this can happen; git itself doesn't produce these, but it is
theoretically possible for the user to do this manually by doing `git rebase
--edit-todo`, or third-party tools could do it too.
We had one user report a crash because of this, so it seems worth fixing it.
Sometimes it takes a while to get PRs accepted upstream, and this blocks our
progress. Since I'm pretty much the only one making changes there anyway, it
makes sense to point to my fork directly.