t: add test_oid option to select hash algorithm

t: add test_oid option to select hash algorithm

In some tests, we have data files which are written with a particular
hash algorithm. Instead of keeping two copies of the test files, we can
keep one, and translate the value on the fly.

In order to do so, we'll need to read both the source algorithm and the
current algorithm, so add an optional flag to the test_oid helper that
lets us look up a value for a specified hash algorithm. This should
not cause any conflicts with existing tests, since key arguments to
test_oid are allowed to contains only shell identifier characters.

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