Move commits to the "COMMIT" mail prefix

Summary:
Fixes T9427. Currently, replies to audits/commits go to "Cxxx", but so do replies to countdowns.

There is non real non-disruptive approach available here and this seems least-bad.

Test Plan:

• Made a comment on a commit.
• Fished the reply-to address out of bin/mail list-oubound + bin/mail show-outbound (it was now "COMMIT...").
• Sent mail to that address.
• Grabbed the raw message and wrote it to mail.txt.
• Ran cat mail.txt | ./scripts/mail/mail_handler.php --process-duplicates.
• Used bin/mail list-inbound + bin/mail show-inbound to verify receipt.
• Saw comment appear on audit.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9427

Differential Revision: https://secure.phabricator.com/D14163