Diffusion - move querying for stable commit name to occur over conduit

Summary: Ref T2784. This is a lower-level one from drequest so it gets the conditional initialization treatment. Consolidated SVN as well even though SVN is issuing database queries; I felt better about the code de-duplication despite the small performance hit when we could just query the DB directly in the SVN case.

Test Plan: browsed around my Phabricator repositories in Mercurial, Git, and SVN flavors. Looked good.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2784

Differential Revision: https://secure.phabricator.com/D5956