title: How can I get the name of the branch that was merged into master in Multibranch pipeline
body: <p>I have a multibranch pipeline job.</p>
  
  <p>When the pipeline runs for the master branch I want to:</p>
  
  <ol>
  <li>Check if this scm push was a merge</li>
  <li>Get the name of the branch that was merged</li>
  </ol>
  
  <p>This way when the master build runs I can see if and which branch branch was merged into it and then do stuff with that info.</p>
  
  <p>It would be nice if this is a built in jenkins feature or if this info can just be read from the GitSCM class</p>