title: Mercurial: how to recover from an interrupted "qpop -a"?
body: <p>I've imported a couple of revisions into the queue and tried to pop them all. Unfortunately, according to some other Mercurial client accessing the same repository, <code>hg qpop -a</code> didn't complete successfully:</p>
  
  <pre><code>&gt; hg qpop -a
  popping 115.diff
  popping 114.diff
  popping 113.diff
  popping 112.diff
  popping 111.diff
  abort: The process cannot access the file because it is being used by
  another process
  C:\Program Files (x86)\Mercurial\library.zip\mercurial\dispatch.py:217:
  DeprecationWarning: use lock.release instead of del lock
  </code></pre>
  
  <p>Now I'm not able anymore to push the patches back. Mercurial always complains about an "unknown node":</p>
  
  <pre><code>&gt; hg qpush -a
  mq status file refers to unknown node b6fb614866f1
  abort: working directory revision is not qtip
  </code></pre>
  
  <p>What does this mean and how can this problem be resolved?</p>