Use the --old path in the diffs produced by the arbitrary diff handler,
just like when using --old with the other diff implementations.

This resolves a crash when running
$ svn diff --old dev/README --new dev/build.conf --summarize

* subversion/libsvn_client/diff.c
  (do_diff_summarize): Provide the right target to the summarize handler.

* subversion/libsvn_client/diff_local.c
  (arbitrary_diff_walker): Use api to detect administrative dir correctly.
  (svn_client__arbitrary_nodes_diff): Use path1 as target.

* subversion/svn/diff-cmd.c
  (summarize_baton_t): New baton type.
  (summarize_xml,
   summarize_regular): Use baton type.

  (svn_cl__diff): Update caller.

* subversion/tests/cmdline/diff_tests.py
  (diff_url_against_local_mods): Extend test to show that in this mode we
    see this as a no change.
  (diff_arbitrary_files_and_dirs): Update expected result.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1428526 13f79535-47bb-0310-9956-ffa450edef68