Function hunks_between 

pub async fn hunks_between(
    root: &Path,
    git_ref: &str,
    tip: Option<&str>,
    wanted: fn(&Path) -> bool,
) -> Result<Vec<Hunk>, GitError>

Hunks between git_ref and an explicit tip, or HEAD when tip is None.

The tip exists for the pre-push hook. git hands a hook the ref being pushed, which need not be the checked-out branch, and reviewing HEAD instead means the pushed code is never seen - so the hook passes the ref’s own oid here.