Revision 1568684

In FSX, unifify the concept of revision and transaction numbers and
call it a "change set".

The mapping is trivial (revs == non-neg change sets, txns are negative
change set numbers) and a few conversion functions are being provided.
As a first step, we use it in fs_x__id_t to represent the noderev_id
unifying txn_id and rev_item.  The API is kept unchanged but the textual
representation changes.

* subversion/libsvn_fs_x/id.h
  (svn_fs_x__change_set_t): Define the new change set number type.
  (SVN_FS_X__INVALID_CHANGE_SET): Define the corresponding NULL value.
  (svn_fs_x__is_revision,
   svn_fs_x__is_txn,
   svn_fs_x__get_revnum,
   svn_fs_x__get_txn_id,
   svn_fs_x__change_set_by_rev,
   svn_fs_x__change_set_by_txn): Declare new conversion & check functions.
  (svn_fs_x__id_part_t): Replace REVISION with CHANGE_SET.

* subversion/libsvn_fs_x/id.c
  (fs_x__id_t): Replace TXN_ID and REV_NUM by NODEREV_ID.
  (svn_fs_x__is_revision,
   svn_fs_x__is_txn,
   svn_fs_x__get_revnum,
   svn_fs_x__get_txn_id,
   svn_fs_x__change_set_by_rev,
   svn_fs_x__change_set_by_txn): Declare new conversion & check functions.
  (txn_id_parse): Drop because we don't have separate txn ids anymore.
  (part_parse): Store ID parts as <number[+,-]change_set> with both
                elements encoded in base36.
  (unparse_id_part): Rename to ...
  (part_unparse): ... this and re-implement.
  (svn_fs_x__id_part_is_root,
   svn_fs_x__id_part_eq): Update due to struct member rename. 
  (svn_fs_x__id_txn_id): Extract the txn id from the noderev change set.
  (svn_fs_x__id_rev_item): Make sure to return a well-defined "invalid" /
                           "unused" struct for non-revision IDs.
  (svn_fs_x__id_rev): Extract the revnum from the noderev change set.
  (svn_fs_x__id_item): Update due to struct member rename.
  (svn_fs_x__id_is_txn): Use the new change set number API.
  (svn_fs_x__id_unparse): Simplify writing just the 3 ID parts.
  (svn_fs_x__id_eq): Adapt to struct change.
  (svn_fs_x__id_check_related): Update check for TXN IDs.
  (svn_fs_x__id_part_compare,
   svn_fs_x__id_txn_create_root, 
   svn_fs_x__id_create_root,
   svn_fs_x__id_txn_create, 
   svn_fs_x__id_rev_create): Adapt to struct update. 
  (svn_fs_x__id_parse): Simplify as we just have to parse 3 id parts.

* subversion/libsvn_fs_x/changes.c
  (binary_change_t): Rename REV_ID to NODEREV_ID.
  (append_change): Update to rename and usage of change set numbers.
  (svn_fs_x__write_changes_container, 
   svn_fs_x__read_changes_container): Adapt to struct member renames. 
  (svn_fs_x__changes_get_list,
   svn_fs_x__changes_get_list_func): Adapt to ID API changes.

* subversion/libsvn_fs_x/transaction.c
  (get_new_txn_node_id,
   svn_fs_x__reserve_copy_id,
   rep_write_contents_close, 
   write_hash_delta_rep,
   get_final_id,
   write_final_rev): Use change set instead of revision numbers and say
                     "noderev_id" instead of "rev_item".

* subversion/libsvn_fs_x/fs_x.c
  (write_revision_zero): Update rev 0 template to use the new ID format.

* subversion/libsvn_fs_x/noderevs.c
  (svn_fs_x__write_noderevs_container,
   svn_fs_x__read_noderevs_container): Adapt to id part member rename. 

* subversion/libsvn_fs_x/verify.c
  (compare_l2p_to_p2l_index,
   compare_p2l_to_l2p_index): Convert from and to change set numbers as
                              required by the APIs and follow renames. 

* subversion/libsvn_fs_x/cached_data.c
  (get_node_revision_body, 
   svn_fs_x__get_node_revision,
   svn_fs_x__get_mergeinfo_count, 
   init_rep_state,
   svn_fs_x__get_representation_length, 
   block_read_changes, 
   block_read_changes_container,
   block_read_noderevs_container, 
   block_read_reps_container,
   block_read): Ditto

* subversion/libsvn_fs_x/pack.c
  (add_item_rep_mapping,
   get_item,
   copy_rep_to_temp,
   copy_node_to_temp,
   compare_p2l_info,
   compare_sub_items,
   compare_p2l_info_rev,
   write_reps_containers, 
   write_l2p_index): Same. 

* subversion/libsvn_fs_x/index.c
  (svn_fs_x__p2l_index_create,
   read_entry): Ditto. 

* subversion/libsvn_fs_x/tree.c
  (x_node_origin_rev): Ditto.