Draw project PHIDs from repositories when evaluating Herald object rules for commits

Summary:
Fixes T12097. In D16413, I simplified this code but caused us to load the commit's projects instead of the repository's projects, which is incorrect.

Normally, commits don't have any project tags when Herald evaluates, so using the commit's projects is generally meaningless.

Test Plan:

• Tagged a repository with #X.
• Created a Herald object rule for commits with #X as the object ("Always ... do nothing.")
• Ran a commit from the repository.
• Before patch: rule failed to evaluate.
• After patch: rule evaluated and passed.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12097

Differential Revision: https://secure.phabricator.com/D17179