Provide basic commit content hooks for Herald

Summary: Ref T4195. This doesn't provide any interesting fields yet (content, affected paths, commit message) but fires the hook correctly.

Test Plan: Added a blocking hook and saw it fire.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4195

Differential Revision: https://secure.phabricator.com/D7789