R1:59db2826247c

Fix an issue with email "Re:" prefix

Summary: See detailed discussion in T1543.

Test Plan:
- Enabled multiplexing.
- Set user A to "enable Re".
- Created a task owned by user B with user A cc'd.
- Verified A got no "Re:" before this patch.
- Applied patch.
- Verified A got "Re:" after this patch.

Reviewers: nh, btrahan, vrana

Reviewed By: nh

CC: aran

Maniphest Tasks: T1543

Differential Revision: https://secure.phabricator.com/D3031