Add an XFail regression test for a file external bug.
Add an XFail regression test for a file external bug.

* subversion/tests/cmdline/externals_tests.py
  (update_modify_file_external): New.
  (test_list): Mark update_modify_file_external XFail.

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion