Add a regression test for issue 4420.  I'm not sure what behaviour
is correct so this test expects the current behaviour.

* subversion/tests/cmdline/externals_tests.py
  (file_external_unversioned_obstruction): New test.
  (test_list): Add new test.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1519712 13f79535-47bb-0310-9956-ffa450edef68