name-branch / topic support

It would be nice to support using topic/branch instead of bookmark. The work started in this !8 (closed) merge request should be able to take care of the export part quite well.

The import part harder because it means injecting metadata in revision we import. So we have to decide "what is part of a branch" which is harder. In git, the content of a branch could be expressed as "everything under the branch head, up to what is under the tracked branch head". However, tracking is not an exchanged information…

So the simplest first step we could have here is assuming everything "tracks" master. This won't only work in simplest case, but is probably enough.