Add XFAIL test for issue 4495, external revision wrong after commit/update.

* subversion/tests/cmdline/externals_tests.py
  (update_external_peg_rev): New XFAIL test.
  (test_list): Add new test.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1590011 13f79535-47bb-0310-9956-ffa450edef68