Finalize fsfs-stats tool by adding histogram outputs
(must frequently changed files by extension, total
file size by extension etc.)

* tools/server-side/fsfs-stats.c
  (histogram_line_t,
   histogram_t,
   extension_info_t): new data aggregation structs
  (fs_fs_t): add global and per-extension histograms
  (add_to_histogram): new utility
  (add_change): add data to the histograms
  (read_noderev): update; count prop representations as well
  (compare_count,
   compare_node_size,
   compare_rep_size,
   get_by_extensions): new data preparation functions
  (print_histogram,
   print_extensions_by_changes,
   print_extensions_by_nodes,
   print_extensions_by_reps,
   print_histograms_by_extension): new stats display function
  (print_stats): show histograms as well

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1449566 13f79535-47bb-0310-9956-ffa450edef68