addGitLabMRComment, update existing comment on subsequent commits

## Issue

It would be nice if subsequent comments in the same merge request would cause the Jenkins plug-in to update the existing comment instead of adding new ones. Of course, controlled with a setting.

### Context
- **Gitlab plugin version**: 1.4.5
- **Gitlab version**: 8.12
- **Jenkins version**: 2.46.3
- **Job type**: Pipeline

### Problem description

I've not yet integrated this plug-in in our build cycle, but during my tests I noticed that merge requests with a lot of commits would get quite spammed by Jenkins. It would be great to be able to update the existing comment, IMO! I'm not sure what the possibilities exist in the GitLab API to do this, though.