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>