RHGitaly: reading GitLab ref state files, implementing RefService.RefExists

This MR provides the core utilities that will more generally allow to resolve all GitLab single revisions into a changeset hash, and then proceeds to implemenent RefExists.

RefExists is both the simplest possible of read requests involving GitLab refs, and a fairly common one (see #128 (closed)), so this is already useful in itself.

With its closures, the high-level utility gitlab::revision::map_full_ref allows to fulfill the RefExists implementation without even converting to binary Nodes. There is in effect nothing calling into Mercurial (hg-core) in this MR.

Try cargo doc --open, the rhgitaly::gitlab module documentation is supposed to be correctly formatted (and fully explained).