Diffusion - fix a bug from T2784 (D5894 to be more specific)

Summary: Fixes T3314. Now you can view the tags for a specific commit.

Test Plan: visited http://phabricator.dev/diffusion/P/tags/master/;9b5f0aa03a5ffc93d5f4fce83fef5efb435097cc and saw a nice, 'no tags' paged. Then I tagged the commit, did a git pull, and reloaded. On reload, the page showed a tag.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T3314

Differential Revision: https://secure.phabricator.com/D6168