Add regression test, for a missing lock on move problem.

* subversion/tests/cmdline/move_tests.py
  (move_to_from_external): New regression test.
  (test_list): Add test.

Found by: schabi

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1551524 13f79535-47bb-0310-9956-ffa450edef68