verify_packfile(): check as many object as possible in a pack

verify_packfile(): check as many object as possible in a pack

verify_packfile() checks for whole pack integerity first, then each
object individually. Once we get past whole pack check, we can
identify all objects in the pack. If there's an error with one object,
we should continue to check the next objects to salvage as many
objects as possible instead of stopping the process.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>