Support "preflight" CI/CD pipeline triggered by Merge Request

Description

This is inspired by Jenkins Gitlab plugin and ElectricFlow preflight build. Before the merge is pushed back to the central Gitlab repo, it would be good to merge and build/test in local repo/workspace on Gitlab runner by Gitlab CI when it's not fast-forward merge.

Proposal

N/A

Links / references

N/A

Documentation blurb

1. Why should someone use it; what's the underlying problem.

   When the merge is not fast-forward, there's chance that the merged commit may cause build failure or functionality problem due to the additional changes on the target branch after the parent commit of feature branch.

2. What is the solution.

   Refer to the Jenkins Gitlab plugin build process upon merge request. See a build log in the attached consoleText.txt.

3. How does someone use this

   Consider to have "merge request" as condition of job in .gitlab-ci.yml similar to "branches" or "tags".