WIP: Comment on commit messages

This code implements (parts of) the functionality discussed in #394. The discussion ended rather quickly, but I decided to go ahead and implement it.

I've been sitting on this code for a couple of months now, and I though I might as well share it. While the pre-squash-and-rebase version of it has been running smoothly on a separate instance, there are some things I'd like to fix with the current implementation. One such thing is to make this functionality opt-in (as mentioned in #394).

I'd love some more feedback on this. Mainly, is there sufficient interest in seeing this feature completed?
Maybe this method belongs to another class, like `CommitCommenter`?