Remove direct calls to LowLevelCommitQuery

Summary: Ref T2783. This cleans up some more of the direct VCS access calls. If the repository is local, this boils down to an in-process call. If not, it uses Conduit to make an intracluster request.

Test Plan: Used reparse.php --message <commit> --trace to observe cluster request.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T2783

Differential Revision: https://secure.phabricator.com/D11253