Teach fsfs-stats tool to identify and print out the largest changes
in a repository.

* tools/server-side/fsfs-stats.c
  (large_change_info_t,
   largest_changes_t): new data structures
  (fs_fs_t): add aggregator for largest changes in repo
  (initialize_largest_changes,
   add_change,
   print_largest_reps): new utlity functions
  (read_noderev): read path as well and forward data to new aggregator
  (read_revisions): also initialize new aggregator
  (print_stats): also print new size stats


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1435805 13f79535-47bb-0310-9956-ffa450edef68