receive-pack: allow a maximum input size to be specified

receive-pack: allow a maximum input size to be specified

Receive-pack feeds its input to either index-pack or
unpack-objects, which will happily accept as many bytes as
a sender is willing to provide. Let's allow an arbitrary
cutoff point where we will stop writing bytes to disk.

Cleaning up what has already been written to disk is a
related problem that is not addressed by this patch.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>