Ui.show_model_changes doesn't reflect reality due to fallback to album

In the case where an album has a field set, and an item has a field of the same name set, and you use beet modify to delete the field from the item, show_model_changes will show it changing to the album’s value, since it’s using format(), which falls back to the album value, rather than showing its removal from the item. Thoughts?

Huh, that does seem bad! Maybe we can work around this to force the change display to use item-level values without overrides?