RepositoryService: Git config get/set/delete methods
Obviously, these don't seem very relevant at first sight for Mercurial, but GitLab can use Git config to store useful information, and it simpler to pretend it is working as if there was a Git repository than to chase all callers.
We could make the Git config a part of existing hgrc files or store it in a separate file, whose format would be completely identical to actual Git config files. I tend to prefer the latter because
- more future-proof, no corner cases requiring special escaping
- we could probably rely on Dulwich for most of the work.
- migration from the Git repo config file to the new file would be trivial.
Dulwich is currently an implicit dependency of HGitaly (via py-heptapod and hg-git), we could make it more direct in anticipation of the very far future where hg-git would be downgraded to a test dependency of HGitaly.