title: Github: Push additional commit to pull request
body: <p>I made a pull request, but now I need to push an additional commit to this pull request. Github says that:</p>
  
  <blockquote>
    <p>You can add more commits to this pull request by pushing to the x4e925e4 branch on myaccount/repo.</p>
  </blockquote>
  
  <p>How do I achieve this via the command line? </p>