Slightly reduce the cost of resolving refs

Summary: Ref T2683. By resolving the stable name earlier, we can save a resolve when viewing branch heads. This is ~100ms in Mercurial, and roughly 25% of page weight. It's less bad in Git.

Test Plan: Saw page cost go down in "Services" tab, particularly for Mercurial browse views.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2683

Differential Revision: https://secure.phabricator.com/D9048