title: emacs ediff-revision for Hg complains about Wrong number of arguments
body: <p>Trying to get ediff-revision working on hg (mercurial-1.8.4) repo using Emacs v23.2 (9.0) on OS X Lion.  I found this <a href="https://stackoverflow.com/questions/5177304/how-do-you-get-ediff-hg-working-in-emacs-23-2-for-mac">answer</a> for the 1st part of my problem but now I'm getting this from Emacs ediff-revision:</p>
  
  <p><em>M-x ediff-revision
  Compare revisions for file (default .hgtags): ~/myrepo.hg/.hgtags
  Revision 1 to compare (default .hgtags's latest revision): 
  Revision 2 to compare (default .hgtags's current state): </em></p>
  
  <p>and I get the following response:</p>
  
  <pre><code>Checking out /Users/username/repo.hg/.hgtags.~tip~...
  apply: Wrong number of arguments: (lambda (file &amp;optional editable rev) "Retrieve a revision of FILE.
  EDITABLE is ignored.
  </code></pre>