Minor improvement to fsfs-stats tool:  Print size histograms for all
extensions that show up in any of the "top 16" lists - not just the
16 most frequent ones.

* tools/server-side/fsfs-stats.c
  (merge_by_extension): new function
  (print_histograms_by_extension): combine extensions from all top-16 lists

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1467148 13f79535-47bb-0310-9956-ffa450edef68