Git push mirroring for native Mercurial repositories

Native repositories won't have an inner Git repository that can be used to mirror to external Git services as people mention in #125 (closed).

In order for that important feature to keep working, we'll simply have to move the conversion to Git inside the push-mirroring asynchronous job.

This is not an immediate concern: at best, native repositories will be available as an option for new projects in %Heptapod 0.17.0 (perhaps only if a given feature flag is activated). In a subsequent release, we would turn it into the default, and yet in another one take care of migrating existing hg-git based repositories to native ones.

I'm assigning this to %Heptapod 1.0 to help not forgetting about it, but in truth, it won't really be needed by that time.