Fix conflicting documentation for svn_(dirent|relpath)_skip_ancestor().

* subversion/include/svn_dirent_uri.h: 
  (svn_dirent_skip_ancestor, svn_relpath_skip_ancestor): They return ""
    not NULL when parent and child are equal.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1519816 13f79535-47bb-0310-9956-ffa450edef68