Add an "Accepted Differential revision" field to Commit and pre-commit Content Herald rules

Summary: Refs T4195. Fixes T3936. You can't currently write rules like "block commits unless they're attached to an **accepted** revision"; allow that.

Test Plan: Pushed commits into a rule with this field, saw it work / not crash.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, mbishopim3

Maniphest Tasks: T3936, T4195

Differential Revision: https://secure.phabricator.com/D7807
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