Support "Repository's projects" field in Commit and Differential Revision rules

Summary: This also cleans up some code a little bit. Most of the gymnastics are to make sure we call needProjectPHIDs() appropriately.

Test Plan: Created new commit and revision rules with this field. Ran commits and revisions through the test console. Field behavior seemed correct.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, dctrwatson

Differential Revision: https://secure.phabricator.com/D7923