use merge commit for build if event is pull request fulfilled

[julioc-p]

Pass merge commit to the git plugin instead of the source commit when a Pull request is merged, thus correcting the behaviour of the pull request merge trigger and avoiding double builds in case of a push event and a pull request merged event at the same time fixing #201

Testing done

Test that the latest commit does in deed uses the merge commit from the payload in case of a pull request merge

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Ensure you have provided tests - that demonstrates feature works or fixes the issue