Implement 128: add a test to merge pipeline for commits behind target branch

• Related issues: #128 (closed)
• Requested reviewers: @kiene

Description

This implements the solution discussed in the issue. However, there is still one case not covered. If merge request passes tests some time earlier and then later commits are merged into main that do not prevent automated merge with this branch, then it probably would be still possible to merge. This probably is something that cannot be prevented via builds using reasonable resources and would be necessarily part of the Gitlab server itself. There are already requests for that in their issue system.