Every selected commit's bitmap was held as a `Vec<bool>` (one byte per object) until the file was assembled, so the writer kept refs x objects bytes alive at once: ~11GB for the kernel (950 refs, 11.8M objects). Encode each bitmap right after its closure walk and keep only the ewah form, which costs at most about one bit per object. The type bitmaps are encoded the same way. Signed-off-by: Seongmin Lee <git@boltless.me>