Modernize Diffusion commitparentsquery

Summary: Ref T4195. Ref T2783. We have an old-school implementation of this; move it into a LowLevel query and make callers all run through Conduit. I need the LowLevel query for hooks, to implement an "is merge commit" Herald rule.

Test Plan:

• Ran query via Conduit for SVN, Mercurial, Git.
• Parsed a commit which closed a revision, attach/closed worked correctly.
• Browsed Diffusion.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4195, T2783

Differential Revision: https://secure.phabricator.com/D7808