show grants permission check for self incorrect

I have a non-super user. They can see their grants IFF they don't specify a user, but if they specify their own id, the permission check fails.

```
db4> show grants;
+--------------------------------------------------------------------+
| Grants for bob@localhost |
+--------------------------------------------------------------------+
| GRANT USAGE ON *.* TO `bob`@`localhost` |
| GRANT SELECT, EXECUTE ON `db4`.* TO `bob`@`localhost` |
| GRANT EXECUTE ON PROCEDURE `db4`.`dolt_fetch` TO `bob`@`localhost` |
+--------------------------------------------------------------------+
3 rows in set (0.00 sec)

db4> show grants for bob@localhost;
command denied to user 'bob'@'localhost'
```

This works as expected in MySQL.