svn cleanup fails on obstructed paths

svn cleanup fails on obstructed paths:

% mkdir repos
% svnadmin create repos
% mkdir wc
% svn co file://`pwd`/repos wc
% cd wc
% svn mkdir foo
% svn ci -m '' foo

#fake a lock:
% touch .svn/lock
% svn st
  L    .

#create an obstruction:
% rm -rf foo/.svn
% svn st
  L    .
~      foo

#attempt cleanup
% svn cleanup
svn: 'foo' is not a working copy directory
Exit 1

% svn st
  L    .
~      foo

At this point it becomes very difficult to recover short of finding and manually
removing all .svn/lock files.

Original issue reported by emiller