On the fsx-id branch:  Introduce a new noderev ID type instead of just using
the part ID to allow for tracking progress.

Add just enough functionality to make svn_fs_x__path_txn_node_rev() use the
new nodedev ID internally to construct the file name.  Since the recent
introduction of unified changeset IDs (instead of txn and revision), the
noderev ID is unique and unchanging.

* subversion/libsvn_fs_x/id.h
  (svn_fs_x__noderev_id_t): Define new ID type for noderevs.
  (svn_fs_x__noderev_id_unparse): Declare similar to svn_fs_x__id_unparse.
  (svn_fs_x__id_noderev_id): Update return value.

* subversion/libsvn_fs_x/id.c
  (svn_fs_x__noderev_id_unparse): Implement similar to svn_fs_x__id_unparse.
  (svn_fs_x__id_noderev_id): Update return value.

* subversion/libsvn_fs_x/util.c
  (svn_fs_x__path_txn_node_rev): Construct the file name from the noderev ID
                                 instead of node and branch ID.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsx-id@1645605 13f79535-47bb-0310-9956-ffa450edef68