delta, packfile: use size_t for delta header sizes

delta, packfile: use size_t for delta header sizes

The delta header decoding functions return unsigned long, which
truncates on Windows for objects larger than 4GB. Introduce size_t
variants get_delta_hdr_size_sz() and get_size_from_delta_sz() that
preserve the full 64-bit size, and use them in packed_object_info()
where the size is needed for streaming decisions.

This was originally authored by LordKiRon <https://github.com/LordKiRon>,
who preferred not to reveal their real name and therefore agreed that I
take over authorship.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>