title: Scan multibranch now builds every (also unchanged) job
body: <p>We have setup multiple multibranch pipeline projects. When clicking on the button 'Scan Multibranch Pipeline Now' all jobs are triggered and built again. It does not matter whether or not there are changes, every job will just start building. </p>
  
  <p>When everything is build and manually scan again, it works as expected and only changed jobs are built. </p>
  
  <p>Running on Jenkins 2.164.2.</p>
  
  <pre><code>Changes detected: master (null -&gt; b95ffd48bf9fd032004e150ae4188cf48d3c28b2)
  Scheduled build for branch: master
  </code></pre>
  
  <p>We expect only the changed branches will be built again.</p>