receive-pack: reorder some code in unpack() This is the preparation for adding --shallow-file to both unpack-objects and index-pack. To sum up: - struct argv_array used instead of const char ** - status/code, ip/child, unpacker/keeper are moved out to function top level - successful flow now ends at the end of the function Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>