Use the FSFS standard parsers for noderevs and representation headers
within the stats code instead of its own (albeit faster) ones.

* subversion/libsvn_fs_fs/stats.c
  (key_matches,
   read_rep_base): Drop as unused now. 
  (parse_representation): Expect a FSFS representation_t to be passed
                          in already sparing us the parsing here.
                          Replace the call to former read_rep_base
                          with invocations of standard parsers.
  (read_noderev): Replace the local parsing logic with a call to the
                  FSFS noderev parser. Update variable usage.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1632849 13f79535-47bb-0310-9956-ffa450edef68