Fix Herald test adapter for commits

Summary:
Fixes T11488. I broke this in D16360, I think by doing a little extra refactoring after testing it.

This code is very old, before commits always needed to have repositories attached in order to do policy checks.

Modernize it by mostly just using the repository which is present on the Commit object, and using the existing edge cache.

Test Plan: Ran a commit through the Herald test adapter.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11488

Differential Revision: https://secure.phabricator.com/D16413