perf(fs/git): only fetch known or newly requested references

[GeorgeMac]

This is an optimization to ensure that we only ever fetch either tracked or newly requested references from the upstream.
Prior to this change, we always fetched all references from the upstream.
Now instead, we always pull the main reference, plus any newly requested refs or previously fetched refs still in the LRU.

I just added an additional change to this, to ensure that when a branch is supplied (only a branch, not a tag or SHA), that we also perform a single branch clone. So that we don't start off with a full clone.

One user reported to us that a new branch in their upstream was unnecessarily fetched and this caused memory pressure on their instance and it OOMed.
Their relevant feedback on this change:

I tracked down the culprit of our sudden increase in resource usage to a random in-development branch of our repo that had just been pushed to the remote.
Since flipt requires the specific git ref for determining which branch / tag to consume, might it be possible to perform a single-ref clone of the repo, rather than cloning all branches?

[markphelps]

Lgtm !