Merge commit label should be removed automatically

[Noratrieb]

When someone pushes merge commits, the merge commits label is added. But when they then push those merge commits away, the label is not removed, making it more annoying for the author or the reviewer to remove it. This is something that could and should be automated too.

cc @pitaj

[Nemo157]

Also if possible minimize the previous comments about the removed merge commits.