t0000: update tests for SHA-256

t0000: update tests for SHA-256

Test t0000 tests the "basics of the basics" and as such, checks that we
have various fixed hard-coded object IDs.  The tests relying on these
assertions have been marked with the SHA1 prerequisite, as they will
obviously not function in their current form with SHA-256.

Use the test_oid helper to update these assertions and provide values
for both SHA-1 and SHA-256.

These object IDs were synthesized using a set of scripts that created
the objects for both SHA-1 and SHA-256 using the same method to ensure
that they are indeed the correct values.

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