Use git native implementation to sync HEAD

We had an issue where ~Geo did not sync the default branch when updating a mirrored repo.

To resolve this, we added a "3-step dance":

1. git remote show geo
2. Parse the HEAD branch in the output
3. Then run git symbolic-ref HEAD refs/heads/whateverremotehead

There was a proposal to have this feature built-in in git, but it was not yet implemented.

I would love to contribute to git, and I'm willing to add it to git. The feature was proposed at https://public-inbox.org/git/20180814214723.GA667@sigill.intra.peff.net/

Of course there is nothing to be done yet on gitaly side, but I created this issue to track some progress.