svnwcsub.py: Improve the wc detection code.  Follow-up to r1429718.

* tools/server-side/svnpubsub/svnwcsub.py
  (WorkingCopy._get_match): Use os.path.join to join paths don't assume / is the path
    separator. Check for an empty .svn dir rather than an empty wc dir since the test
    is redundent otherwise.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1432779 13f79535-47bb-0310-9956-ffa450edef68