Use RepositoryGraphCache to service diffusion.lastmodifiedquery

Summary:
Ref T2683. At least locally, browse views are now nearly instantaneous, even in Mercurial. We also fall back to what we were doing before if we miss or take too long, so this shouldn't make things very much worse even in extreme cases.

For a local hg repo, the time we spend pulling browse stuff has dropped from ~3,000ms to ~20ms. This is probably atypical, but not completely crazy or rigged or anything.

Test Plan: Viewed Git, Subversion and Mercurial repositories and observed dramatically better performance in Git and Mercurial as they took advantage of the cache.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley, jhurwitz

Maniphest Tasks: T2683

Differential Revision: https://secure.phabricator.com/D9047