t/helper/test-tool: implement sha1-unsafe helper

t/helper/test-tool: implement sha1-unsafe helper

With the new "unsafe" SHA-1 build knob, it is convenient to have a
test-tool that can exercise Git's unsafe SHA-1 wrappers for testing,
similar to 't/helper/test-tool sha1'.

Implement that helper by altering the implementation of that test-tool
(in cmd_hash_impl(), which is generic and parameterized over different
hash functions) to conditionally run the unsafe variants of the chosen
hash function, and expose the new behavior via a new 'sha1-unsafe' test
helper.

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