t: allow testing different hash algorithms via environment

t: allow testing different hash algorithms via environment

To allow developers to run the testsuite with a different algorithm than
the default, provide an environment variable, GIT_TEST_DEFAULT_HASH, to
specify the algorithm to use. Compute the fixed constants using
test_oid. Move the constant initialization down below the point where
test-lib-functions.sh is loaded so the functions are defined.

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