This test produces pseudo-collisions and tests git diff's behavior with
them, and is therefore sensitive to the hash in use. Update the test to
compute the collisions for both SHA-1 and SHA-256 using appropriate
constants. Move the heredocs inside the setup block so that all of the
setup code can be tested for failure.
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>