Allow username changes which modify letter case to go through as valid

Summary:
Fixes T13446. Currently, the validation logic here rejects a rename like "alice" to "ALICE" (which changes only letter case) but this is a permissible rename.

Allow collisions that collide with the same user to permit this rename.

Also, fix an issue where an empty rename was treated improperly.

Test Plan:
  - Renamed "alice" to "ALICE".
    - Before: username collision error.
    - After: clean rename.
  - Renamed "alice" to "orange" (an existing user). Got an error.
  - Renamed "alice" to "", "!@#$", etc (invalid usernames). Got sensible errors.

Maniphest Tasks: T13446

Differential Revision: https://secure.phabricator.com/D20890

src/applications/people/xaction/PhabricatorUserUsernameTransaction.php

@@ -71,21 +71,30 @@ final class PhabricatorUserUsernameTransaction
       }
 
       if (!strlen($new)) {
-        $errors[] = $this->newRequiredError(
+        $errors[] = $this->newInvalidError(
-          pht('New username is required.'),  $xaction);
+          pht('New username is required.'),
+          $xaction);
       } else if (!PhabricatorUser::validateUsername($new)) {
         $errors[] = $this->newInvalidError(
-          PhabricatorUser::describeValidUsername(), $xaction);
+          PhabricatorUser::describeValidUsername(),
+          $xaction);
       }
 
       $user = id(new PhabricatorPeopleQuery())
         ->setViewer(PhabricatorUser::getOmnipotentUser())
         ->withUsernames(array($new))
         ->executeOne();
-
       if ($user) {
+        // See T13446. We may be changing the letter case of a username, which
+        // is a perfectly fine edit.
+        $is_self = ($user->getPHID() === $object->getPHID());
+        if (!$is_self) {
           $errors[] = $this->newInvalidError(
-          pht('Another user already has that username.'), $xaction);
+            pht(
+              'Another user already has the username "%s".',
+              $new),
+            $xaction);
+        }
       }
 
     }