func (w *Writer) prepareForWrite(t plumbing.ObjectType, size int64) {
w.pending = size
w.hasher = plumbing.NewHasher(t, size)
w.multi = io.MultiWriter(w.zlib, w.hasher)
}
func (r *Reader) prepareForRead(t plumbing.ObjectType, size int64) {
r.hasher = plumbing.NewHasher(t, size)
r.multi = io.TeeReader(r.zlib, r.hasher)
}