R1:af300016b678

Fix exception trying to rename user to their previous username

Summary:
When trying to rename a user, properly handle when the new username is the old username to avoid an exception.

```
EXCEPTION: (PhabricatorApplicationTransactionNoEffectException) Transactions have no effect:
- Transaction (of type "user.rename") has no effect. at [<phorge>/src/applications/transactions/editor/PhabricatorApplicationTransactionEditor.php:2823]
```

Closes T15795

Test Plan:
* As an admin, go to `/people/manage/123/`…