Resolve a regression reported by <GezwiZ> in #svn IRC, see r1600311.

* subversion/libsvn_client/externals.c
  (svn_client__handle_externals): Fix removal of empty ancestor directories
    of externals that are removed. Properly handle deleted and non existing
    directories.

* subversion/tests/cmdline/externals_tests.py
  (update_deletes_file_external): Remove XFail marker.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1600323 13f79535-47bb-0310-9956-ffa450edef68