Add an upgrade from 1.7 regression test that fails when working
copy exclusive locking is enabled.

* subversion/tests/cmdline/upgrade_tests.py
  (upgrade_1_7_dir_external): New test.
  (test_list): Add new test.

* subversion/tests/cmdline/upgrade_tests_data/upgrade_1_7_dir_external.tar.bz2:
  New.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1572102 13f79535-47bb-0310-9956-ffa450edef68