Function try_write_streaming_pack 

pub fn try_write_streaming_pack(
    repo: &Repository,
    wants: &[ObjectId],
    haves: &[ObjectId],
    source_shallow: &HashSet<ObjectId>,
    have_shallow: &HashSet<ObjectId>,
    filter: Option<&ObjectFilter>,
    opts: &PackBuildOptions,
    progress: &mut dyn FnMut(PackProgress),
    w: &mut dyn Write,
) -> Result<Option<PackStats>>

Write a PACK stream to w using bitmap reachability and verbatim reuse when possible.

Returns None when reuse is unavailable (caller should stream via crate::pack_objects::PackObjects).