message: don't merge abstract class into its parent if it have concrete method
diff: @@ -66,6 +66,18 @@ void check_dont_merge_list(
  // removable).
  if (find_parent->second != STRICT && can_delete(child_cls) &&
  can_rename_if_ignoring_blanket_keepnames(child_cls)) {
  + if (is_abstract(child_cls)) {
  + for (auto method : child_cls->get_vmethods()) {
  + if (method->get_code()) {
  + return;
  + }
  + }
  + for (auto method : child_cls->get_dmethods()) {
  + if (method->get_code()) {
  + return;
  + }
  + }
  + }
  (*mergeable_to_merger)[child_cls] = parent_cls;
  }
  }
  