Another go at the relpath_is_canonical check.  As it turns out, it is
much faster to check for "." and "//" elements in seperate runs due to
the simpler patterns that we check for.

* subversion/libsvn_subr/dirent_uri.c
  (relpath_is_canonical): rework the scanner part

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1489828 13f79535-47bb-0310-9956-ffa450edef68