hash: add an algo member to struct object_id

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>