CVS: branch support

Created by: LawnGnome

This is split from #23757 (closed).

We need to be able to select branches of interest in CVS and replicate them to Git as best we can. We may need to use a similar approach to the tag algorithm in #24935 (closed) to fake commits at times, but we should also try not to where the CVS branches represent coherent whole-repo states that can be directly translated to Git's branching model.