Function try_bitmap_reachable_for_upload 

pub fn try_bitmap_reachable_for_upload(
    odb: &Odb,
    wants: &[ObjectId],
    haves: &[ObjectId],
    filter: Option<&ObjectFilter>,
    shallow_grafts: &HashSet<ObjectId>,
) -> Result<Option<(ReachableSet, ReachableSet)>>

Bitmap reachability for upload-pack when shallow grafts and filters allow it.

Returns the send-side ReachableSet and the peer-held ReachableSet.