R1:36d9908e6cd1

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 | .