Note they key_or() may call tree_delete(), which will cause the weight
asserts to be checked. In order to avoid them from firing, update key1
tree's weight after we've changed key1->some_local_child->next_key_part.

Having done that, do we still need this at the function end:

  /* Re-compute the result tree's weight. */
  key1->update_weight_locally();
?