Add a new XFail test for a problem with 'svn diff' and actual-only nodes.

$ svn status
!     C iota
      >   local file missing, incoming file edit upon merge
      Summary of conflicts:
        Tree conflicts: 1
$ svn diff
svn: E155010: The node 'iota' was not found.

(The actual error prints an absolute path, but was shortened for this
log message).

*  subversion/tests/cmdline/diff_tests.py
   (diff_missing_tree_conflict_victim, test_list): New test.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1483391 13f79535-47bb-0310-9956-ffa450edef68