Unbreak 'svn diff C:/' on Windows and/or 'svn diff /' on *nix.

* subversion/libsvn_wc/diff_local.c
  (ensure_state): Don't recurse endlessly when the anchor path is a root
    dirent.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1541635 13f79535-47bb-0310-9956-ffa450edef68