CommitService: implement CommitLanguages

This is about programming or markup language detection, as opposed to natural languages.

It will be a tough one: in Gitaly, it is handled by the Ruby side, using the GitHub linguist gem, which could well be very Git specific.

Including Ruby gems in HGitaly would be a pain (as it is with Gitaly actually, because of side effects between bundles), and it's not clear if could just feed diffs and files extracted from Mercurial programming to primitives purely about language detection.

On the other hand, using anything else will bring us visible changes in results that may be undesirable, and reimplementing everything is probably too much work in the short term.