Convert object iteration callbacks to struct object_id

Convert object iteration callbacks to struct object_id

Convert each_loose_object_fn and each_packed_object_fn to take a pointer
to struct object_id.  Update the various callbacks.  Convert several
40-based constants to use GIT_SHA1_HEXSZ.

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