Finalize work on 'svnfsfs stats' for FSFS f6.

Replace the revision trailer parser with the standard one provided by FSFS.
We no longer need to read the whole revision contents at once.

* subversion/libsvn_fs_fs/stats.c
  (get_content,
   read_revision_header): No longer needed.
  (read_phys_revision): Extract the trailer and parse it via FSFS.  Calculate
                        the length of the changed paths list string here now.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1639355 13f79535-47bb-0310-9956-ffa450edef68