On the fsfs-format7 branch:  Switch the txn_id member of representation_t
from C string to ID part struct.  Update users and their interfaces to
use ID part structs for txn IDs, too.  Introduce and use utility functions
to check whether a txn ID struct is used (equivalent to a non-NULL txn_id)
and to reset one to unused state.

This is another patch of the "numerical IDs" patch series.

* subversion/libsvn_fs_fs/fs.h
  (representation_t): switch txn_id from string to struct

* subversion/libsvn_fs_fs/id.h
  (svn_fs_fs__id_txn_used,
   svn_fs_fs__id_txn_reset): declare API functions to test & reset txn IDs

* subversion/libsvn_fs_fs/id.c
  (svn_fs_fs__id_txn_used,
   svn_fs_fs__id_txn_reset): implement new API functions

* subversion/libsvn_fs_fs/cached_data.c
  (open_and_seek_transaction): convert txn_id to string
  (open_and_seek_representation,
   create_rep_state_body,
   svn_fs_fs__rep_chain_length,
   build_rep_list,
   get_dir_contents,
   svn_fs_fs__get_proplist): use new API to test / reset txn

* subversion/libsvn_fs_fs/rep-cache.c
  (svn_fs_fs__walk_rep_reference,
   svn_fs_fs__get_rep_reference): ditto

* subversion/libsvn_fs_fs/low_level.c
  (svn_fs_fs__parse_representation,
   svn_fs_fs__unparse_representation): ditto
  (read_rep_offsets): parse string

* subversion/libsvn_fs_fs/temp_serializer.c
  (serialize_representation,
   deserialize_representation): update and simplify

* subversion/libsvn_fs_fs/util.h
  (path_txn_sha1): switch txn_id parameter from string to struct

* subversion/libsvn_fs_fs/util.c
  (path_txn_sha1): update; temporarily convert to string here

* subversion/libsvn_fs_fs/transaction.c
  (store_sha1_rep_mapping): update API caller
  (set_uniquifier): drop TXN_ID parameter; update
  (svn_fs_fs__set_entry,
   get_shared_rep,
   rep_write_contents_close,
   svn_fs_fs__set_proplist): update callers
  (set_representation,
   write_final_rev,
   svn_fs_fs__delete_node_revision): use new API to test / reset txn


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1454784 13f79535-47bb-0310-9956-ffa450edef68