`rows affected` for `alter` differs from MySQL

dolt
```
tmp/main*> alter table set_table modify column c0 varchar(100);
Query OK, 0 rows affected (0.01 sec)
```

mysql
```
mysql> alter table set_table modify column c0 varchar(100);
Query OK, 3 rows affected (0.03 sec)
Records: 3  Duplicates: 0  Warnings: 0
```
