On the fsfs-format7 branch: Change ID from "rev/offset" to "rev/item"
and update callers.  Pre-format 7 repos will use offset+1 for the item
index value.

* subversion/libsvn_fs_fs/id.h
  (svn_fs_fs__id_offset): replaced by ...
  (svn_fs_fs__id_item): ... this one
  (svn_fs_fs__id_rev_create): update signature

* subversion/libsvn_fs_fs/id.c
  (id_private_t): replace 'offset' with 'item'
  (svn_fs_fs__id_offset): replaced by ...
  (svn_fs_fs__id_item): ... this one
  (svn_fs_fs__id_unparse,
   svn_fs_fs__id_eq,
   svn_fs_fs__id_txn_create,
   svn_fs_fs__id_rev_create,
   svn_fs_fs__id_copy,
   svn_fs_fs__id_parse): update

* subversion/libsvn_fs_fs/cached_data.c
  (open_and_seek_revision): simplify using the new item->offset mapping
  (get_cached_node_revision_body,
   set_cached_node_revision_body
   get_node_revision_body): update

* subversion/libsvn_fs_fs/recovery.c
  (recover_find_max_ids,
   svn_fs_fs__find_max_ids): additional item->offset conversion required

* subversion/libsvn_fs_fs/transaction.c
  (allocate_item_index): new function, WIP
  (write_final_rev): assign item indexes
  (commit_body): update; write revision trailer for old formats only


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1432703 13f79535-47bb-0310-9956-ffa450edef68