Fix a variable typo which prevents metadata from attaching to @mention Differential comments

Summary: If you @mention several users, at least one of which is already CC'd,
we unset all the CCs and don't attach the "Added CCs: ..." block to the comment.

Test Plan: @mentioned two users, one of whom was already CC'd.

Reviewers: jungejason, tuomaspelkonen, aran

Reviewed By: jungejason

CC: aran, jungejason

Differential Revision: 827