R1:9c938701c3dd

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

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