title: git stash reporting: '' is not a stash reference
body: <p>Not quite sure what's happened, but git stash seems to be in a bad place.</p>
  
  <pre><code>% git stash list
  stash@{0}: filter-branch: rewrite
  stash@{1}: filter-branch: rewrite
  stash@{2}: On mysolr: start mysolr stuff
  </code></pre>
  
  <p>Is OK, and <code>git show stash@{0}</code> works fine.  But:</p>
  
  <pre><code>% git stash drop
  '' is not a stash reference
  % git stash pop
  '' is not a stash reference
  % git stash drop stash@{0}
  'stash@{0}' is not a stash reference
  </code></pre>
  
  <p>I've used git stash plenty in the past and not come across this.  I had recently rewritten history to remove a file from history before publishing to github.  The command I ran then was</p>
  
  <pre><code>git filter-branch --force --index-filter \
    'git rm --cached --ignore-unmatch FILENAME' \
    --prune-empty --tag-name-filter cat -- --all
  </code></pre>
  
  <p>Any ideas how to fix the stash?</p>