On the fsfs-format7 branch:  Use containers to store the change lists
in packed rev files.  This gives a ~10x compression for this data.

This patch does not support reading packed change lists, yet (only
passively as part of block-read).

* subversion/libsvn_fs_fs/index.h
  (SVN_FS_FS__ITEM_TYPE_CHANGES_CONT): declare new item type

* subversion/libsvn_fs_fs/cached_data.c
  (block_read_changes_container): new reader function
  (block_read): allow for containers and handle changes containers

* subversion/libsvn_fs_fs/pack.c
  (auto_pad_block): factor out from ...
  (copy_items_from_temp): ... this
  (write_changes_container,
   get_block_left,
   write_changes_containers): new functions to create changes containers
  (write_l2p_index): fix a pointer reference
  (pack_range): call the new packing function

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1465038 13f79535-47bb-0310-9956-ffa450edef68