Add a regression test for the issue fixed in r31685.
Add a regression test for the issue fixed in r31685.

* subversion/tests/cmdline/diff_tests.py
  (diff_external_diffcmd): New test.
  (test_list): Run the new test, should pass.


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@871762 13f79535-47bb-0310-9956-ffa450edef68

YueLinHo/Subversion,wbond/subversion,wbond/subversion,wbond/subversion,YueLinHo/Subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,YueLinHo/Subversion,wbond/subversion,YueLinHo/Subversion,YueLinHo/Subversion,wbond/subversion,wbond/subversion,YueLinHo/Subversion