Load commit branches and tags by AJAX

Summary:
Each query takes over 2 seconds in FBCODE.
I didn't find a way how to speed them up.
There's also no easy way how to parallelize them at least.
So AJAX is the last instance.

Test Plan: Loaded commit with one branch and no tag.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3112