On the fsx-id branch:  Remove also the second svn_fs_id_t from FSX'
transaction_t struct.

* subversion/libsvn_fs_x/fs.h
  (transaction_t): Replace the base root noderev-ID with the base rev.

* subversion/libsvn_fs_x/transaction.h
  (svn_fs_x__get_txn_ids): Rename to ...
  (svn_fs_x__get_base_rev): ... this.  Make it return a revision instead
                            of a FS API ID.

* subversion/libsvn_fs_x/transaction.c
  (svn_fs_x__get_txn): Simplify transaction_t initialization.
  (svn_fs_x__open_txn,
   svn_fs_x__get_txn_ids): Update.

* subversion/libsvn_fs_x/dag.c
  (svn_fs_x__dag_txn_base_root): Update caller; construct root noderev-ID
                                 on demand.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsx-id@1647806 13f79535-47bb-0310-9956-ffa450edef68