Fixed diff generator when several identical tables or columns were renamed

Fixed diff generator when two tables have several columns with identical DDL and several of them were renamed.

Sample table schema:
```yml
propel:
  page:
    id:                 ~
    image_url_1:        { type: varchar, size: 255 }
    image_url_2:        { type: varchar, size: 255 }
    image_url_3:        { type: varchar, size: 255 }
```

When renaming field image_url_2 -> image_url_4 and image_url_3 -> image_url_5 ```propel:diff``` generates the following broken UpSQL:
```sql
ALTER TABLE `page` CHANGE `image_url_2` `image_url_4` VARCHAR(255);

ALTER TABLE `page` CHANGE `image_url_3` `image_url_4` VARCHAR(255);

ALTER TABLE `page`
    ADD `image_url_5` VARCHAR(255) AFTER `image_url_4`;
```

Reverse DownSQL is broken too:
```sql
ALTER TABLE `page` CHANGE `image_url_4` `image_url_2` VARCHAR(255);

ALTER TABLE `page` CHANGE `image_url_4` `image_url_3` VARCHAR(255);

ALTER TABLE `page` DROP `image_url_5`;
```

This happens because the same added field can be used several times when detecting renaming.

This patch fixes this wrong behavior.