Use Application PHIDs for commits

Summary: Ref T2716. Ref T2715. Move CMIT to use Application PHIDs. Nothing too special here, but I consolidated some code into DiffusionCommitQuery. Depends on D6514.

Test Plan: Browsed Diffusion. Browsed Differential/Maniphest with linked commits. Used jump nav; used phid.lookup and phid.query. Used remarkup for Git and SVN repos. Grepped for PHID_TYPE_CMIT.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2715, T2716

Differential Revision: https://secure.phabricator.com/D6515