Trigger merge request hook when source updated

**What does this MR do?**
Allow trigger merge request web hook when source project updated

**Are there points in the code the reviewer needs to double check?**

- https://github.com/Bugagazavr/gitlabhq/blob/ac0f1c24be125a0bc49fbee502503626077c94d6/app/models/merge_request.rb#L113
- https://github.com/Bugagazavr/gitlabhq/blob/ac0f1c24be125a0bc49fbee502503626077c94d6/app/services/git_push_service.rb#L54-L59

**Why was this MR needed?**
Allow detect source changes in MR for many external systems