t6030: make test work with SHA-256

t6030: make test work with SHA-256

Compute several object ID values instead of hard-coding them, and use
test_oid_to_path to cleanly produce a path for an object.

Note that the bisect code which is tested here remains sensitive to the
hash algorithm in use because it uses the object ID to disambiguate
between two equidistant commits.  Fortunately, SHA-1 and SHA-256
disambiguate identically in the cases we care about, so there is no need
to modify the test to accommodate this situation.  However, if a further
hash algorithm change occurs, this test may require some restructuring.

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