manifest: help pytype to understant `writesubtrees`'s `getnode` type

Since we provide a default, the return of `_lazydirs.get` is cannot be None. We
help pytype to understand that.

mercurial/manifest.py

@@ -1407,7 +1407,9 b' class TreeManifest:'
             ld = m._lazydirs.get(d)
             if ld:
                 return ld[0]
-            return m._dirs.get(d, emptytree)._node
+            tree = m._dirs.get(d, emptytree)
+            assert tree is not None  # helps pytype
+            return tree._node
 
         # let's skip investigating things that `match` says we do not need.
         visit = match.visitchildrenset(self._dir[:-1])