Function difft::diff::sliders::fix_nested_slider_prefer_outer

fn fix_nested_slider_prefer_outer<'a>(
    node: &'a Syntax<'a>,
    change_map: &mut ChangeMap<'a>
)

When we see code of the form (old-1 (novel (old-2))), prefer treating the outer delimiter as novel, so (novel ...) in this example.