Resolve issue #4422 "During update an incoming authz node where a file external
exists causes an assertion", by tweaking the specific absent handing in the
update editor. (Follow up to r1520065, which added the test for this issue)

* subversion/libsvn_wc/update_editor.c
  (absent_node): Handle obstructing file externals next to the existing
    directory externals handling.

* subversion/tests/cmdline/authz_tests.py
  (authz_file_external_to_authz): Remove XFail marker and add issue number.
    Tweak comments.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1520745 13f79535-47bb-0310-9956-ffa450edef68