Convert the verify_pack callback to struct object_id

Convert the verify_pack callback to struct object_id

Make the verify_pack_callback take a pointer to struct object_id.
Change the pack checksum to use GIT_MAX_RAWSZ, even though it is not
strictly an object ID.  Doing so ensures resilience against future hash
size changes, and allows us to remove hard-coded assumptions about how
big the buffer needs to be.

Also, use a union to convert the pointer from nth_packed_object_sha1 to
to a pointer to struct object_id.  This behavior is compatible with GCC
and clang and explicitly sanctioned by C11.  The alternatives are to
just perform a cast, which would run afoul of strict aliasing rules, but
should just work, and changing the pointer into an instance of struct
object_id and copying the value.  The latter operation could seriously
bloat memory usage on fsck, which already uses a lot of memory on some
repositories.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>