Add XFail test showing that diff of a directory fails without rights of reading
the ancestor directory on svn and dav.

This is caused by rooting the ra operation one level higher than the root,
to allow the diff to properly describe directory replacements, which would be
impossible if the directory is used as the root.

* subversion/tests/cmdline/merge_authz_tests.py
  (diff_unauth_parent): New test. XFail for svn
  (test_list): Add diff_unauth_parent.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1569265 13f79535-47bb-0310-9956-ffa450edef68