run-command: mark unused async callback parameters

run-command: mark unused async callback parameters

The start_async(), etc, functions need a "proc" callback that conforms
to a particular interface. Not every callback needs every parameter
(e.g., the caller might not even ask to open an input descriptor, in
which case there is no point in the callback looking at it). Let's mark
these for -Wunused-parameter.

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