Add reproduction recipe for a way to trigger the assertion on line 3043 of
update_editor.c. This error has been reported quite a few times on the users
list. E.g. in http://svn.haxx.se/users/archive-2012-08/0288.shtml
(But also today in the 'Problem' thread)

* subversion/tests/cmdline/authz_tests.py
  (authz_file_external_to_authz): New testcase that reproduces an assertion
    in the update editor.
  (test_list): Add authz_file_external_to_authz.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1520065 13f79535-47bb-0310-9956-ffa450edef68