t/helper/test-hash.c: use unsafe_hash_algo()

t/helper/test-hash.c: use unsafe_hash_algo()

Remove a series of conditionals within the shared cmd_hash_impl() helper
that powers the 'sha1' and 'sha1-unsafe' helpers.

Instead, replace them with a single conditional that transforms the
specified hash algorithm into its unsafe variant. Then all subsequent
calls can directly use whatever function it wants to call without having
to decide between the safe and unsafe variants.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>