Diffusion - move commit branches query to conduit
Summary: Ref T2784.
Test Plan: loaded up a mercurial and a git commit. verified branches showed up correctly in object detail panel
Reviewers: epriestley
Reviewed By: epriestley
CC: aran, Korvin
Maniphest Tasks: T2784
Differential Revision: https://secure.phabricator.com/D5958