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 stuck with the error in T12406.
• Applied the change; worker now succeeded.

Reviewers: chad

Reviewed By: chad

Subscribers: alexmv

Maniphest Tasks: T12406

Differential Revision: https://secure.phabricator.com/D17507