On the fsx-id branch:  Add getters for node and copy id to the DAG node.
This allows us to replace all outside references to svn_fs_x__dag_get_id.

* subversion/libsvn_fs_x/dag.h
  (svn_fs_x__dag_get_node_id,
   svn_fs_x__dag_get_copy_id): Declare new internal API functions.
  (svn_fs_x__dag_related_node): Declare new convenience function.

* subversion/libsvn_fs_x/dag.c
  (svn_fs_x__dag_get_node_id,
   svn_fs_x__dag_get_copy_id,
   svn_fs_x__dag_related_node): Implement them.

* subversion/libsvn_fs_x/tree.c
  (get_copy_inheritance,
   make_path_mutable,
   x_node_relation,
   merge_changes,
   x_closest_copy): Use the new DAG API to replace calls to
                    svn_fs_x__dag_get_id.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsx-id@1645847 13f79535-47bb-0310-9956-ffa450edef68