Function reachable_closure 

pub fn reachable_closure(
    odb: &Odb,
    roots: &[ObjectId],
    stop: &HashSet<ObjectId>,
    skip_missing: bool,
    shallow_grafts: &HashSet<ObjectId>,
) -> Result<HashSet<ObjectId>>

Compute the full object closure reachable from roots, stopping descent into any object already present in stop.