Don't rebuild commits that already have a commit status

It seems unnecessary to rebuild all of the commits in a pull request when pushing a single new commit to that PR, since all of the previous commits have been built and have commit statuses set by Leeroy. Is there a reason to do it this way?

I'm willing to contribute a patch to this, it looks like I'd need to make changes here: https://github.com/litl/leeroy/blob/master/leeroy/github.py#L55