hash: use uint32_t for object_id algorithm

hash: use uint32_t for object_id algorithm

We currently use an int for this value, but we'll define this structure
from Rust in a future commit and we want to ensure that our data types
are exactly identical.  To make that possible, use a uint32_t for the
hash algorithm.

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