Function diff_listings 

pub fn diff_listings(
    before: &BTreeMap<String, String>,
    after: &BTreeMap<String, String>,
) -> Diff

The binary rung: diff two file listings by content id alone.

Each listing maps a workspace-relative path to the content id of the bytes at that path. A path present on one side only is Added/Removed; a path on both sides with a different id is Changed; an unchanged id yields no delta. Deltas come back sorted by path, so the same inputs always produce the same diff. Moved is never produced here — it needs the engine’s rename detection, not a content-id comparison.