Prepare larger packet buffer for upload-pack protocol.

Prepare larger packet buffer for upload-pack protocol.

The original side-band support added to the upload-pack protocol used the
default 1000-byte packet length.  The pkt-line format allows up to 64k, so
prepare the receiver for the maximum size, and have the uploader and
downloader negotiate if larger packet length is allowed.

Signed-off-by: Junio C Hamano <junkio@cox.net>