t4042: choose the built-in hash outside of a repo

t4042: choose the built-in hash outside of a repo

Right now, the built-in default hash is always SHA-1, but that will
change in a future commit.  Instead of assuming that operating outside
of a repository will always use SHA-1, provide constants for both
algorithms and then simply ask test_oid for the built-in hash instead,
which will always be correct.

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