R1:dec8acd8038c

Add very basic tag support to Diffusion

Summary: Lists the 25 most recent tags on the "Repository" page.

Test Plan: Looked at a git repository with a tag, saw it. Looked at HG/SVN repos, they didn't break.

Reviewers: davidreuss, 20after4, btrahan, vrana, jungejason

Reviewed By: davidreuss

CC: aran

Maniphest Tasks: T1130

Differential Revision: https://secure.phabricator.com/D2255