To make it possible in a following commit to move packet
reading and writing functions into a Packet.pm module,
let's refactor these functions, so they don't handle
printing debug output and exiting.
While at it let's create packet_required_key_val_read()
to still handle erroring out in a common case.
Signed-off-by: Christian Couder <chriscool@tuxfamily.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>