Make replacing_by emit slightly earlier

The `replacing_by` signal is mainly used to update the cached scene
root when replacing the scene root. Emit this signal a little early
to ensure that certain flags for the editor are not obsolete during
entering tree.

doc/classes/Node.xml

@@ -1067,7 +1067,7 @@
  <param index="0" name="node" type="Node" />
  <description>
  Emitted when this node is being replaced by the [param node], see [method replace_by].
- This signal is emitted [i]after[/i] [param node] has been added as a child of the original parent node, but [i]before[/i] all original child nodes have been reparented to [param node].
+ This signal is emitted [i]before[/i] [param node] is added as a child of the original parent node.
  </description>
  </signal>
  <signal name="tree_entered">

scene/main/node.cpp

@@ -3015,17 +3015,16 @@ void Node::replace_by(Node *p_node, bool p_keep_groups) {
  _clean_up_owner();
  }
 
- Node *parent = data.parent;
- int index_in_parent = get_index(false);
+ emit_signal(SNAME("replacing_by"), p_node);
 
+ Node *parent = data.parent;
  if (data.parent) {
+ int index_in_parent = get_index(false);
  parent->remove_child(this);
  parent->add_child(p_node);
  parent->move_child(p_node, index_in_parent);
  }
 
- emit_signal(SNAME("replacing_by"), p_node);
-
  while (get_child_count()) {
  Node *child = get_child(0);
  remove_child(child);