Comment on Merge Request
<!--IssueSummary start-->
<details>
<summary>
Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards.
</summary>
- [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=29896)
</details>
<!--IssueSummary end-->
### Summary
Merge requests are not updated after creation if additional commits are pushed.
### What is the current behavior?
Comments are only allowed on file level.
### What is the expected *correct* behavior?
Comments for the whole merge request.
### Problem to solve
Sometimes I want to comment something of a merge request that is not related to a specific file.
### Intended users
* Sasha (Software Developer)
* Delaney (Development Team Lead)
### Further details
* comment on file and directory structure
* comment about missing files
* comment on issues related to many files, that would have to be redundant or incomplete otherwise
### Proposal
Add a possibility to add top-level comments for a merge request.
### Permissions and Security
Same as for existing comments.
### Testing
Same as for existing comment feature.
### What does success look like, and how can we measure that?
It is possible to write top-level comments for a merge request.
### Links / references
issue