Add a regression test (XFail) for issue #4366 ("client SEGFAULTs
diffing a repos rev in which an empty file was added").

* subversion/tests/cmdline/diff_tests.py
  (diff_repos_empty_file_addition): New test.
  (test_funcs): Add reference to new test.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1482969 13f79535-47bb-0310-9956-ffa450edef68