OperationService: implement UserFFBranch

This is the gRPC method behind fast-forward merges (a simple phase change on Mercurial repositories).

It should be even simpler than UserSquash, and the performance improvement would be useful already, as it will replace several hg subprocesses with a single HGitaly call that loads the repository only once.

It would also be the first to carry over a MR iid, needed to switch permissions checks to merge mode in the common Python layer calling back the Rails app (UserSquash did not need it).