Add repository to audit emails

Summary: This is vaguely related to T5791. Add a "REPOSITORY" header to audit emails so that they can be filtered in Gmail.

Test Plan: Commented on an audit and used ./bin/mail show-outbound to inspect outbound email.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: fabe, Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12997