packfile: use get_be64() for large offsets

packfile: use get_be64() for large offsets

The pack-index version 2 format uses two 4-byte integers in
network-byte order to represent one 8-byte value. The current
implementation has several code clones for stitching these integers
together.

Use get_be64() to create an 8-byte integer from two 4-byte integers
represented this way.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>