t5703: make test work with SHA-256

t5703: make test work with SHA-256

This test used an object ID which was 40 hex characters in length,
causing the test not only not to pass, but to hang, when run with
SHA-256 as the hash.  Change this value to a fixed dummy object ID using
test_oid_init and test_oid.

Furthermore, ensure we extract an object ID of the appropriate length
using cut with fields instead of a fixed length.

Signed-off-by: brian m. carlson <redacted>
Signed-off-by: Junio C Hamano <redacted>