- Item_in_subselect::inject_in_to_exists_cond() should not call 
  ((Item_cond*)join->conds)->argument_list()->concat(join->cond_equal->current_level)
  as that makes two lists share their tail, and the cond_equal list
  will end up containing non-Item_equal objects when substitute_for_best_equal_field()
  walks through join->conds and replaces all Item_equal objects with Item_func_eq objects.
  - So, instead of using List::concat(), manually copy entries from one list to another.