Add test for issue #4464.

* subversion/tests/cmdline/diff_tests.py
  (is_absolute_url): new utility function
  (make_diff_header): Don't add .../ prefix to absolute URLs.
  (diff_repos_wc_copies, test_list): new test


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1561237 13f79535-47bb-0310-9956-ffa450edef68