pass TIME_DATE_NOW to approxidate future-check

pass TIME_DATE_NOW to approxidate future-check

The approxidate functions accept an extra "now" parameter to
avoid calling time() themselves. We use this in our test
suite to make sure we have a consistent time for computing
relative dates. However, deep in the bowels of approxidate,
we also call time() to check whether possible dates are far
in the future. Let's make sure that the "now" override makes
it to that spot, too, so we can consistently test that
feature.

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