Add square brackets around new revision review requests

Summary:
These didn't get covered when we added square brackets to the rest of the emails
(so gmail can thread them properly).

Test Plan:
Created a local diff, got an email with brackets.

Reviewed By: tuomaspelkonen
Reviewers: rm, tuomaspelkonen, jungejason, aran
CC: aran, epriestley, tuomaspelkonen
Differential Revision: 256