Function difft::diff::unchanged::split_mostly_unchanged_toplevel

fn split_mostly_unchanged_toplevel<'a>(
    lhs_nodes: &[&'a Syntax<'a>],
    rhs_nodes: &[&'a Syntax<'a>]
) -> Vec<(Vec<&'a Syntax<'a>>, Vec<&'a Syntax<'a>>)>

Split out top-level lists that are largely the same.

This is important in cases where we have two adjacent lists that have a small number of changes.

; old
(1 2 3 4) (a b c d)

; new
(1 2 novel 3 4) (a b novel-2 c d)

By splitting out these mostly-unchanged lists, we can substantially further shrink them.