Support Merge Request diff comment for GitLab

[chimame]

Problems and suggestions

I want to correspond to diff comment in Merge Request of GitLab

Countermeasure

Currently only commit comment is supported, but we would like to insert comments in diff comment if possible. By doing so, the development team can easily grasp all indications by checking diff.

Fortunately gitlab corresponds to it.

https://github.com/NARKOZ/gitlab/blob/b543f256c54be60a228346ba9f9932b62ffb6eba/lib/gitlab/client/merge_requests.rb#L184-L210

However, the version of gem must be 4.4.0 or higher. This change may cause some GitLab versions to be truncated as well.

[mbobin]

Yes, we need this. I could work on a pull request for it.

[Strnadj]

This is closed in https://github.com/prontolabs/pronto/pull/351 ?