R1:9dbde24dda72

Manually prune Git working copy refs instead of using "--prune", to improve "Fetch Refs" behavior

Summary:
Ref T13448. When "Fetch Refs" is configured:

- We switch to a narrow mode when running "ls-remote" against the local working copy. This excludes surplus refs, so we'll incorrectly detect that the local and remote working copies are identical in cases where the local working copy really has surplus refs.
- We rely on "--prune" to remove surplus local refs, but it only prunes refs matching the…