Repo aware monitors: add gitserver client ResolveRevisions

Created by: camdencheek

This adds ResolveRevisions to the gitserver client. This resolves hashes, refs, and globs to hashes. This will be used to resolve what exact commits will be searched in advance of a commit search so we can exclude those commits in subsequent runs.

Test plan

Added a pretty integration-ey test that makes me feel good about this.