R1:65de9e9f5e7f

Ignore "Auditors: author" when inferring auditors from commit messages

Summary:
Fixes T12406. When importing commits, we automatically add auditors if the message lists "Auditors: username".

If the list of auditors includes the commit author, this edit fails because you can't audit your own commits (previously, you sometimes could and/or we didn't validate).

Instead, just ignore "Auditors: author".

Test Plan:
- Made a commit with "Auditors: epriestley".
- Pushed it.
- Saw the HeraldWorker get…