Resolve the diff issue identified in r1569265, and extend the regression test
to show why we do anchor the operation a bit higher than the diff.

* subversion/libsvn_client/diff.c
  (diff_prepare_repos_repos): Detect the case where we try to parent the
    session on an unreadable ancestor and avoid this case by reparenting
    to the node itself.

* subversion/tests/cmdline/merge_authz_tests.py
  (diff_unauth_parent): Remove XFail and extend test to ensure that we
    show as much of diffs as possible given the rights a user has.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1569290 13f79535-47bb-0310-9956-ffa450edef68