Add support for PackRefs RPC

While working with a customer importing a lot of GitHub pull requests, GitLab appears to create several keep-around refs per pull request imported. We saw 50,000+ keep-around refs created for the repository.

While investigating slow pushes, we noticed that git-receive-pack will load all keep-around refs, even if the hideRefs hides these refs. Packing these refs frequently is therefore important to performance.

We already have GitGarbageCollect, RepackIncremental, and RepackFull, but we may not want to issue a full git gc. I think it makes sense to add PackRefs with the option of --all.