run-command: mark error routine parameters as unused

run-command: mark error routine parameters as unused

After forking but before exec-ing a command, we install special
error/warn/die handlers in the child. These ignore the error messages
they get, since the idea is that they shouldn't be called in the first
place.

Arguably they could pass along that error message _in addition_ to
saying "error() should not be called in a child", but since the whole
point is to avoid any conflicts on stdio/malloc locks, etc, we're better
to just keep these simple. Seeing them trigger is effectively a bug, and
the developer is probably better off grabbing a stack trace.

But we do want to mark the functions so that -Wunused-parameter doesn't
complain.

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