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.
ffccfd6e42