title: svn checkout fails with "E160006: No such revision" after relocation and upgrade
body: <p>I have an SVN repository that just recently started complaining on checkouts of a particular folder that the checkout fails due to "E160006: No such revision XXX", where XXX is the number of a particular revision.  The missing revision appears to date from 6 years ago.  Checkout of other folders works fine.  I am not sure what caused this, but several weeks ago I switched to using a backup of my original repository that I had been maintaining using svnsync.  I also recently updated my (ubuntu linux) operating system, which lead subversion to request that I execute "svn upgrade" on my working copy, which I attempted, but I had to stop the upgrade because of a missing VPN connection, which lead to a completely non-functional working copy (but I don't see how problems with a working copy could be related to this problem with checkout; more likely the problem is in the other direction).</p>
  
  <p>Is there anything I can do to fix this problem (for example, to recover the missing revisions or just instruct subversion to ignore them)?  I have the original repository that the "svnsync repo" was generated from, but this original repository lacks recent commits.</p>
  
  <p>Thanks,
  David</p>