pack-bitmap: don't perform unaligned memory access

pack-bitmap: don't perform unaligned memory access

The preceding bitmap entries have a 1-byte XOR-offset and 1-byte flags,
so their size is not a multiple of 4. Thus the name-hash cache is only
guaranteed to be 2-byte aligned and so we must use get_be32 rather than
indexing the array directly.

Signed-off-by: James Clarke <jrtc27@jrtc27.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>