R1:cce786a89f97

Prevent mentions from matching "derp@derp"

Summary: See task / comment.
Test Plan:
https://secure.phabricator.com/file/view/PHID-FILE-f337d112ffe053fdea7d/
Reviewed By: tomo
Reviewers: mroch, tomo
CC: aran, tomo
Differential Revision: 614