Client support for PackRefs

Problem

Fetching refs when they aren't packed is slow. Sine we already have an RPC for packing refs, we should be more aggressive in how we pack refs.

Further details

In the context of reducing disk IO in Gitaly, this is good change because it is easy to back port and is much simpler than the cache invalidation problem of gitaly#1631 (closed)

Proposal

gitlab-ce should regularly call PackRefs in order to keep the refs packed

Links / references