In FSX, replace the txn_id / rev+item members in representation_t with
a single change set based id part.

Most of the code churn is simply adding / removing conversions between
the old dual representation and the new change set based struct.

* subversion/libsvn_fs_x/fs.h
  (representation_t): Replace the txn/rev dual members with the unified
                      ID part struct.

* subversion/libsvn_fs_x/fs_x.c
  (svn_fs_x__noderev_same_rep_key): Simplify based on the unified struct.

* subversion/libsvn_fs_x/low_level.c
  (svn_fs_x__parse_representation,
   read_rep_offsets,
   svn_fs_x__unparse_representation): Update representation_t struct access
                                      and remove txn special casing.

* subversion/libsvn_fs_x/noderevs.c
  (binary_representation_t): Switch the container-internal representation
                             to change set based id part as well.
  (store_representation,
   get_representation,
   write_reps,
   read_reps): Update struct accessors and conversions.

* subversion/libsvn_fs_x/cached_data.c
  (open_and_seek_transaction,
   open_and_seek_representation,
   create_rep_state_body,
   svn_fs_x__rep_chain_length,
   build_rep_list,
   svn_fs_x__get_contents,
   svn_fs_x__try_process_file_contents,
   get_dir_contents,
   locate_dir_cache,
   svn_fs_x__get_proplist): Update representation_t struct access and
                            ID conversion.

* subversion/libsvn_fs_x/pack.c
  (copy_node_to_temp,
   write_reps_containers): Ditto.

* subversion/libsvn_fs_x/reps.c
  (svn_fs_x__reps_add_base): Same.

* subversion/libsvn_fs_x/rep-cache.c
  (rep_has_been_born,
   svn_fs_x__walk_rep_reference,
   svn_fs_x__get_rep_reference, 
   svn_fs_x__set_rep_reference): Same. 

* subversion/libsvn_fs_x/transaction.c
  (store_sha1_rep_mapping,
   svn_fs_x__set_entry,
   choose_delta_base,
   rep_write_get_baton,
   get_shared_rep,
   rep_write_contents_close,
   svn_fs_x__set_proplist,
   write_hash_delta_rep,
   write_final_rev,
   svn_fs_x__delete_node_revision): Ditto

* subversion/libsvn_fs_x/verify.c
  (verify_walker): Ditto.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1568792 13f79535-47bb-0310-9956-ffa450edef68