gh-44871: Only walk ancestors when the new child has children

A node without children cannot be an ancestor, so testing this first
keeps appending leaf nodes linear in the depth of the tree.

Also fix references which do not resolve on main.

Author: serhiy-storchaka

Doc/library/xml.dom.minidom.rst

@@ -259,7 +259,7 @@ This reflects information in the XML document that is not of general
 utility to most DOM users.
 
 .. versionchanged:: next
-   :class:`EntityReference` is now implemented.
+   :class:`~xml.dom.EntityReference` is now implemented.
    Note that the parser expands entity references,
    so they only occur in a document if created explicitly.
 

Doc/library/xml.dom.rst

@@ -816,8 +816,8 @@ EntityReference Objects
 .. class:: EntityReference
 
    Represents an entity reference in the XML document.
-   It is a subclass of :class:`Node`.
-   The name of the referenced entity is its :attr:`nodeName`.
+   It is a subclass of :class:`!Node`.
+   The name of the referenced entity is its :attr:`Node.nodeName`.
    Its children are the replacement text of the entity,
    and are read-only.
 

Lib/xml/dom/minidom.py

@@ -93,7 +93,9 @@ def _check_new_child(self, newChild):
         if newChild is self:
             raise xml.dom.HierarchyRequestErr(
                 "%s cannot be child of itself" % repr(self))
-        if _is_ancestor(newChild, self):
+        # A node without children cannot be an ancestor, and testing this
+        # first keeps appending leaf nodes linear in the depth of the tree.
+        if newChild.childNodes and _is_ancestor(newChild, self):
             raise xml.dom.HierarchyRequestErr(
                 "%s is an ancestor of %s" % (repr(newChild), repr(self)))