Use delta islands in RepackFull and GarbageCollect

Closes #1382 (closed)

This changes the behavior of the RepackFull and GarbageCollect RPC's to make sure that we use create a delta island around the objects reachable via refs/heads and refs/tags. The purpose of this is to increase delta chain reuse during git clone and git fetch.

This won't do anything until we have Git 2.20+ in Omnibus https://gitlab.com/gitlab-org/gitlab-ce/issues/54255 but nothing breaks when we merge this already.

Also, use of delta islands is behind a feature flag and off by default.