title: What branch the pipeline executes on gitlab merge request? (source/target/result of the merge)
body: <p>We are trying to build a CI with Gitlab / Gitlab+jenkins. The flow we desire to have is:</p>
  
  <ol>
  <li>Update (commit + push) in feature branch</li>
  <li>Merge request from feature branch to master</li>
  <li>Merge request should check for conflicts &amp; then trigger CI job (GitLab pipeline or Jenkins job)</li>
  <li>The CI job should operate only on the result of the merge</li>
  <li>Reviewer should see the status of the CI job and in case of success he can approve the merge in GitLab GUI</li>
  </ol>
  
  <p>Currently we see that the CI job operates on the source branch (to be merged) and not on the merged branch (after conflicts check). Is it possible to execute the CI on the merged branch without pushing to the master in case of CI success? (push only after reviewer approves)</p>
  
  <p>Thanks</p>