unit-tests: ignore unused argc/argv

unit-tests: ignore unused argc/argv

All of the unit test programs have their own cmd_main() function, but
none of them actually look at the argc/argv that is passed in.

In the long run we may want them to handle options for the test harness.
But we'd probably do that with a shared harness cmd_main(), dispatching
to the individual tests. In the meantime, let's annotate the unused
parameters to avoid triggering -Wunused-parameter.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>