Continue work on 'svnfsfs stats' for FSFS f6.

Use the FSFS standard parser to read the changed paths list and get
the number entries in it.

* subversion/libsvn_fs_fs/stats.c
  (revision_info_t): Remove obsolete member.
  (get_change_count): Replace by ...
  (get_phys_change_count): ... this.  Use FSFS standard functions to read
                           the change list and count its entries.
  (read_phys_revision): Update caller.

  (get_log_change_count): The former get_change_count.
  (read_log_rev_or_packfile): Call it here.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1639352 13f79535-47bb-0310-9956-ffa450edef68