made DiffusionHomeController use commit summaries

Summary: Latest commit in repositories are displayed together with their summaries if available

Test Plan:
Diffusion did not crash - a good sign

commit summaries also only appear on Diffusion Home //only// - as expected

Reviewers: epriestley, btrahan, chad

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5117