Backport some packing code cleanup from FSX to FSFS.  None of it should
change the behavior.

* subversion/libsvn_fs_fs/pack.c
  (): Mention that we use separate regions for directories and files.
  (initialize_pack_context): Formatting fixes only.
  (close_pack_context): Use SVN_VA_NULL instead of plain NULL in variable
                        parameter lists.
  (sort_reps,
   sort_items,
   write_l2p_index): Use svn_sort__array instead of qsort for APR arrays.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1596299 13f79535-47bb-0310-9956-ffa450edef68