git-clean: Fix error message if clean.requireForce is not set.

git-clean: Fix error message if clean.requireForce is not set.

It was distracting to see this error message:

     clean.requireForce set and -n or -f not given; refusing to clean

even though clean.requireForce was not set at all. This patch distinguishes
the cases and gives a different message depending on whether the
configuration variable is not set or set to true.

While we are here, we also divert the error messages to stderr.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>