title: Jenkinsfile check if build triggered by push or merge request
body: <p>Right now I have configured jenkins gitlab connection and gitlab webhook to trigger jenkins jobs.</p>
  
  <p>I would like to create Jenkinsfile where some tasks are executed only if build was triggered by push and some others only if build was triggered by merge request. I use pipeline jobs. Is that possible?</p>
  
  <p>BUILD_CAUSE variable shows nothing.</p>
  
  <p>Thanks.</p>