Since `copy_clone()` preserved IDs, `traverse_pre_order_ids()` can be
called on the old tree, without collecting into a `Vec`.
Then we can also `copy_clone()` only if there's actually a change, and
also only call `push_tree()` in that case.
(Once the `LazyCell::get()` stabilization is released, we could use
that here, but `Option::get_or_insert_with()` may be more readable
anyway.)
With this, `cleanup_drag()` should be pretty low-cost, so we shouldn't
have to worry about whether or not it's redundant.
de7c78accd