Fire GitLab pre-receive hook directly from Mercurial

We currently push as Administrator (root) to the inner auxiliary Git repository. It was useful for the early prototype but has now several drawbacks:

• a special acces rule allows that (just solved a merge conflict with it while preparing the heptapod-0-8 branch)
• it goes over SSH with a dedicated key. There are of course no two instances using the same key (it's generated on first startup), but now that we're going to open SSH access onto Mercurial repos, we still don't want it to lay around.
• there's probably some incurred latency, but I don't expect it to be the performance bottleneck
• source of divergence of rules between hg push and work done from the interface. We'll have to be careful with that, because isomorphic enforcement may be perceived as regression by our users.

This will involve several projects: py-heptapod, hg-git an heptapod-docker projects