RHGitaly ability to read GitLab ref files

The GitLab state files are maintained by heptapod/py-heptapod and map all refs visible to GitLab to the corresponding changeset Node IDs.

Although HGitaly has a fallback in case they are missing, it is actually not used when called for the repository of a Mercurial native Project (but HGitaly is also used in some cases on legacy, hg-git based Projects, e.g. for archive and backup production).

Once this capability is added, many requests can be implemented without even completing hg-core.