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

src/applications/differential/editor/comment/DifferentialCommentEditor.php

@@ -351,7 +351,7 @@ class DifferentialCommentEditor {
         $current_ccs = array_fill_keys($current_ccs, true);
         foreach ($mention_ccs as $key => $mention_cc) {
           if (isset($current_ccs[$mention_cc])) {
-            unset($mention_ccs);
+            unset($mention_ccs[$key]);
           }
         }
       }