Add regression test for issue #4519.

* subversion/tests/cmdline/externals_tests.py
  (switch_relative_externals): New function.
  (test_list): Add switch_relative_externals.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1625500 13f79535-47bb-0310-9956-ffa450edef68