Resolve assertion triggered by the test added in r1604736, by properly
detecting a canonical relpath, with special behavior where we are about to
use it.

* subversion/libsvn_client/diff.c
  (diff_repos_wc): Properly calculate anchor and target when the target is ".."

* subversion/tests/cmdline/diff_tests.py
  (diff_parent_dir): Extend test to verify an actual change via both . and .. .
    Remove XFail marker.

* subversion/tests/libsvn_subr/dirent_uri-test.c
  (test_relpath_is_canonical): Add test value, verifying current behavior.
    "../.." is verified as canonical, so we can't just redefine "..".
    Relpaths just don't have a magic value to select the parent dir.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1604782 13f79535-47bb-0310-9956-ffa450edef68