Distinguish between mainline commits and in-branch commits

[Jonathan Lange]

It would be nice if 'bzr stats' had some way of producing output that matched only commits to trunk. According to 'bzr stats' against lp:launchpad, I am the most prolific human committer, but that's largely because I commit to my local branches very often during the course of development. If it were measured by branches landed, then I would be further down the ranks.