gitserver: Restore repo pinning functionality

Created by: eseliger

When we switched to this helper method, we lost repo pinning as this method doesn't take it into consideration. Since we have it in the settings, I'd rather restore it or completely remove it, but not keep it broken. Who knows who relies on it. Currently, pinned repos would most likely just be unreachable. Current behavior: The gitserver.Client uses the pinning map, and directs traffic to the defined gitserver, but the gitserver would not recognize that repo as "owned by itself" hence it would never clone it / unclone it.

Test plan

Test suite gotta tell us if this breaks gitserver.