pack-objects: use streaming interface for reading large loose blobs

pack-objects: use streaming interface for reading large loose blobs

git usually streams large blobs directly to packs. But there are cases
where git can create large loose blobs (unpack-objects or hash-object
over pipe). Or they can come from other git implementations.
core.bigfilethreshold can also be lowered down and introduce a new
wave of large loose blobs.

Use streaming interface to read/compress/write these blobs in one
go. Fall back to normal way if somehow streaming interface cannot be
used.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>