Follow-up to r1570053: fix ctypes-python binding test.

* subversion/bindings/ctypes-python/test/wc.py
  (test_diff): Per the r1570053 change we no longer report revision 0 for
    non-existent paths.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1573927 13f79535-47bb-0310-9956-ffa450edef68