Several callers use fixed buffers for storing the pack
object header, and they've picked 10 as a magic number. This
is reasonable, since it handles objects up to 2^67. But
let's give them a constant so it's clear that the number
isn't pulled out of thin air.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>