Stop all object mentions from matching after "@"

Summary:
Fixes T9479. Currently, `@aaaaaaaa` may try to match as a commit hash, and `@C123456` may try to match as a Countdown reference. These should only match as user mentions.

Prevent object mention rules from matching after `@`. We already prevent them after `-` and `#`, and already prevented the username rule after `@` (i.e., preventing `@@user`).

Test Plan:
Created some "interesting" users locally and `@mentioned` them:

{F850779}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9479

Differential Revision: https://secure.phabricator.com/D14186

src/applications/diffusion/remarkup/__tests__/DiffusionCommitRemarkupRuleTestCase.php

@@ -121,6 +121,25 @@ final class DiffusionCommitRemarkupRuleTestCase extends PhabricatorTestCase {
           ),
         ),
       ),
+
+      // After an "@", we should not be recognizing references because these
+      // are username mentions.
+      'deadbeef' => array(
+        'embed' => array(
+        ),
+        'ref' => array(
+          array(
+            'offset' => 0,
+            'id' => 'deadbeef',
+          ),
+        ),
+      ),
+      '@deadbeef' => array(
+        'embed' => array(
+        ),
+        'ref' => array(
+        ),
+      ),
     );
 
     foreach ($cases as $input => $expect) {

src/infrastructure/markup/rule/PhabricatorObjectRemarkupRule.php

@@ -209,13 +209,14 @@ abstract class PhabricatorObjectRemarkupRule extends PhutilRemarkupRule {
       $boundary = '\\B';
     }
 
-    // The "(?<![#-])" prevents us from linking "#abcdef" or similar, and
-    // "ABC-T1" (see T5714).
+    // The "(?<![#@-])" prevents us from linking "#abcdef" or similar, and
+    // "ABC-T1" (see T5714), and from matching "@T1" as a task (it is a user)
+    // (see T9479).
 
     // The "\b" allows us to link "(abcdef)" or similar without linking things
     // in the middle of words.
 
-    return '((?<![#-])'.$boundary.$prefix.'('.$id.')(?:#([-\w\d]+))?(?!\w))u';
+    return '((?<![#@-])'.$boundary.$prefix.'('.$id.')(?:#([-\w\d]+))?(?!\w))u';
   }