Display arbitrary groups of branches in the commit view.

[aharrison24]

We can already choose between showing All / Local / Current branch in the commit view. That's useful, but what would be super-useful is the ability to display an arbitrary subset of branches.

This would help a lot in situations where you're studying a pair of branches for a potential merge or rebase, but there are a bunch of other branches in the vicinity which are just adding noise to the picture. The existing filters don't allow sufficient granularity for this currently.