Merged commits bypass email preferences

[cespare]

Discovered an interesting bug/consequence-of-the-data-model today. Say you have a saved search for 'repos: foo' (implicitly this only covers the master branch). Then you make commit c1 to the branch B. When barkeep picks up this commit, it checks to see who has a saved search that includes c1 (probably nobody, unless someone is watching branch B or 'branches: all'). Then if you later merge c1 to master, it's not a new commit and so no email is ever sent.