Fix special case in file comparison code posted here:
http://svn.haxx.se/dev/archive-2011-02/0392.shtml

* subversion/libsvn_subr/io.c
  (contents_identical_p): handle special case

Found by: danielsh

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/integrate-io-improvements@1072297 13f79535-47bb-0310-9956-ffa450edef68