merge: do not pass unused opt->value parameter

merge: do not pass unused opt->value parameter

The option_parse_strategy() callback does not look at opt->value;
instead it calls append_strategy(), which manipulates the global
use_strategies array directly. But the OPT_CALLBACK declaration assigns
"&use_strategies" to opt->value.

One could argue this is good, as it tells the reader what we generally
expect the callback to do. But it is also bad, because it can mislead
you into thinking that swapping out "&use_strategies" there might have
any effect. Let's switch it to pass NULL (which is what every other
"does not bother to look at opt->value" callback does). If you want to
know what the callback does, it's easy to read the function itself.

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