Provide a fast path for resolving repository refs

Summary:
Ref T2783. With service-oriented calls, we take a larger performacne hit than necessary resolving refs.

Instead of resolving refs over the wire, try to resolve them from the database first. This can resolve almost all refs (commit hashes, branch and tag names).

This can't resolve weird refs like master~50, and obviously can't resolve invalid refs. In those cases we'll go back to the old logic, call diffusion.resolverefs, and end up with the right result.

Test Plan:

• Browsed repositories in Diffusion.
• Verified that service repositories no longer make unnecessary diffusion.resolverefs calls for common refs (branch names, commit hashes).
• Resolved refs like master~50, saw call to underlying VCS and correct result.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2783

Differential Revision: https://secure.phabricator.com/D11476