There is no point in storing stats counters, item sizes and sums as signed
values.  Use ui64 throughout.

* subversion/include/private/svn_fs_fs_private.h
  (svn_fs_fs__largest_changes_t): File sizes shall be ui64, SIZE member is
                                  still limited to what fits into RAM.
  (svn_fs_fs__large_change_info_t,
   svn_fs_fs__histogram_line,
   svn_fs_fs__rep_pack_stats,
   svn_fs_fs__representation,
   svn_fs_fs__node_stats_t,
   svn_fs_fs__stats_t): Counters, sizes and sums shall be ui64.

* subversion/libsvn_fs_fs/stats.c
  (rep_stats_t,
   revision_info_t): Store file sizes and counters as ui64.
  (get_change_count): Return count in ui64.

* subversion/svnfsfs/stats-cmd.c
  (print_rep_stats,
   print_largest_reps,
   print_histogram,
   print_extensions_by_changes,
   print_extensions_by_nodes,
   print_extensions_by_reps,
   print_stats): Counters are unsigned now, thus use the formatter
                 functions for ui64.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1639335 13f79535-47bb-0310-9956-ffa450edef68