Previously we searched commits based on the author email address, but
this isn't really right: if I commit something from a contributor, I'm
still responsible for MFCing it, so really we should be filtering on the
committer.

Add a new --committer option to filter results by committer email
address, defaulting to the user.email value in the git config.

Keep the --author option, but don't filter by author unless the option
is explicitly specified.

Reported by: des