R1:a9704428ffa5

In Audit, use repository identities to prevent author-auditors

Summary:
See PHI2015. Diffusion attempts to prevent a commit's author from being made an auditor, but currently uses an out-of-date method for identifying the author.

Use the modern ("Repository Identity" aware) method instead.

Test Plan:
- Authored a commit as user "X", mapped to my account.
- Pushed/imported/discovered it.
- Changed the identity mapping for "X" from my account to a different account.
- Tried to add myself as an…