Separate GitLab branches logic from hg-git

This is the natural follow-up to heptapod#351 (closed): to prepare for %HGitaly2: fully native, we will need to keep maintaining the GitLab branches state file and send GitLab hooks, but transactions on Mercurial native repositories won't go through hg-git any more.

A majority of the methods of HeptapodGitHandler will have to move to some mixin class, that will be written with no hg_git assumptions, and some transitional methods of the same kind as we've done in the Rails app: gitlab_sha_from_sha andsha_from_gitlab_sha.

This can be done in the 0.18.0 time frame