title: Git: How to return from 'detached HEAD' state
body: <p>If one would checkout a branch:</p>
  
  <p><code>git checkout 760ac7e</code> </p>
  
  <p>from e.g. <code>b9ac70b</code>, how can one go back to the last known head <code>b9ac70b</code> without knowing its SHA1?</p>