Missing Index On versioncontrol_item_revisions

It would appear that quite often we join versioncontrol_item_revisions against versioncontrol_operations to get full commit info. This join is usually done ON vc_op_id, but there is no index on versioncontrol_item_revisions.vc_op_id. We should add one, as without it we tend to have a full table scan on item_revisions. Example:

Before

| id | select_type | table                         | type   | possible_keys | key          | key_len | ref                                            | rows | Extra       |
|  1 | SIMPLE      | versioncontrol_repositories   | index  | PRIMARY       | PRIMARY      | 4       | NULL                                           | 9016 | Using index |
|  1 | SIMPLE      | versioncontrol_item_revisions | ref    | repo_id_path  | repo_id_path | 4       | git_dev.versioncontrol_repositories.repo_id    |   48 | Using where |
|  1 | SIMPLE      | versioncontrol_operations     | eq_ref | PRIMARY       | PRIMARY      | 4       | git_dev.versioncontrol_item_revisions.vc_op_id |    1 | Using index |

After

| id | select_type | table                         | type   | possible_keys     | key     | key_len | ref                                            | rows | Extra       |
|  1 | SIMPLE      | versioncontrol_item_revisions | range  | repo_id_path,test | test    | 4       | NULL                                           |   14 | Using where |
|  1 | SIMPLE      | versioncontrol_operations     | eq_ref | PRIMARY           | PRIMARY | 4       | git_dev.versioncontrol_item_revisions.vc_op_id |    1 | Using index |
|  1 | SIMPLE      | versioncontrol_repositories   | eq_ref | PRIMARY           | PRIMARY | 4       | git_dev.versioncontrol_item_revisions.repo_id  |    1 |             |