hash: add an algo member to struct object_id

Now that we're working with multiple hash algorithms in the same repo, it's best if we label each object ID with its algorithm so we can determine how to format a given object ID. Add a member called algo to struct object_id. Performance testing on object ID-heavy workloads doesn't reveal a clear change in performance. Out of performance tests t0001 and t1450, there are slight variations in performance both up and down, but all measurements are within the margin of error. Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>