Provide an initial version of what a FSFS private API stats function
would look like.  The main change over the previous read_revisions()
is that the FS struct is passed passed in instead of being created
locally and the result is only a stats struct not the full query object.

* subversion/svnfsfs/stats-cmd.c
  (fs_open): Remove. Superseded by create_query().
  (read_revisions): Take an existing query instance.  Move the construction
                    code to the new construction utils.
  (create_stats): New construction utility.  Code mainly taken from
                  former read_revisions().
  (create_query): New construction utility.  Code taken from former 
                  fs_open() and read_revisions().

  (svn_fs_fs__get_stats): First version of the new interface function.
  (subcommand__stats): Update caller.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1632681 13f79535-47bb-0310-9956-ffa450edef68