title: How to fix a Mercurial repo that fails updating because points to missing revision in subrepo?
body: <p>Our mercurial repository gets stuck when trying to update to an old branch which has a subrepo  / revision pair that doesn't exist anymore.</p>
  
  <pre><code>abort: unknown revision '22e9bb6a6cd98be85b995f632b2f72d6298f9354'!
  </code></pre>
  
  <p>Is there any way to tell Mercurial to update to a revision or branch but not attempt to update the subrepos?</p>