API: Comment on merge request diff

Enhancing the comment API to have the possibility to comment on a merge-request diff would be a gread benefit. This would open the world for some interesting integrations like HoundCI for GitLab (https://github.com/houndci/hound).

What does the core team think about such an enhancement?

I would be willing to contribute with an merge request if there's some interest in this. I've already made some proof-of-concept code by taking a look into the implementation in !137 (merged).