Enable the arbritrary file diff support for 'svn diff --summarize'.

* subversion/libsvn_client/diff.c
  (do_diff_summarize): Handle working-working diffs using the new diff_local
    handling.

* subversion/tests/cmdline/diff_tests.py
  (diff_arbitrary_same): New test.
  (test_list): Add test.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1428426 13f79535-47bb-0310-9956-ffa450edef68