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