Move "most recent commit" and "commit count" into DiffusionRepositoryQuery

Summary: Ref T2625. `DiffusionHomeController` currently runs these queries inline. Move them into `DiffusionRepositoryQuery`. Prepareds for ApplicationSearch.

Test Plan: Loaded `/diffusion/`, saw the same content as before.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2625

Differential Revision: https://secure.phabricator.com/D6914