title: Clean builds with Multibranch Workflow
body: <p>Using Multibranch Workflow, the command to check out looks like</p>
  
  <pre><code>checkout scm
  </code></pre>
  
  <p>I can't find a way to tell Jenkins to perform a clean checkout. By "clean," I mean it should remove all files from the workspace that aren't under version control.</p>