On the fsx-id:  Since the txn root noderev-ID is trivial to construct,
we don't need to store it in the transaction_t object. 

* subversion/libsvn_fs_x/fs.h
  (transaction_t): Remove root_id element.

* subversion/libsvn_fs_x/transaction.h
  (svn_fs_x__get_txn_ids): Don't return the root id anymore.

* subversion/libsvn_fs_x/transaction.c
  (svn_fs_x__get_txn): The root noderev ID is only a temporary now and
                       of our cheep internal ID type.
  (svn_fs_x__get_txn_ids): One value less to return.

* subversion/libsvn_fs_x/dag.c
  (svn_fs_x__dag_txn_base_root): Simplify caller.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsx-id@1647804 13f79535-47bb-0310-9956-ffa450edef68