Support all kinds of merges, not just pull requests.

Merges also work as a nice toplevel block of change even if they don't
have as strong of an appearance on GitHub.  Their messages are often
unhelpful, but that isn't necessarily the case so it makes sense to
still look at them.

This partially addresses #23.
Merge pull request #49 from nubs/merge-changes

Support all kinds of merges, not just pull requests.