message: use NodeData::combine
diff: @@ -285,11 +285,12 @@ impl Tree {
  /// Link to the root node
  pub fn root(&self) -> EntryLink { self.root }
  - /// Reference to the root ndoe
  + /// Reference to the root node.
  pub fn root_node(&self) -> Result<IndexedNode, Error> {
  self.resolve_link(self.root)
  }
  + /// If this tree is empty.
  pub fn is_empty(&self) -> bool {
  self.stored_count == 0
  }
  @@ -359,29 +360,12 @@ mod tests {
  }
  }
  - fn node(start_height: u64, end_height: u64) -> NodeData {
  - NodeData {
  - consensus_branch_id: 1,
  - subtree_commitment: [0u8; 32],
  - start_time: 0,
  - end_time: 0,
  - start_target: 0,
  - end_target: 0,
  - start_sapling_root: [0u8; 32],
  - end_sapling_root: [0u8; 32],
  - subtree_total_work: 0.into(),
  - start_height: start_height,
  - end_height: end_height,
  - shielded_tx: 7,
  - }
  - }
  -
  fn initial() -> Tree {
  let node1: Entry = leaf(1).into();
  let node2: Entry = leaf(2).into();
  let node3 = Entry {
  - data: node(1, 2),
  + data: NodeData::combine(&node1.data, &node2.data),
  kind: EntryKind::Leaf,
  };
  