By default, check-ignore does not list tracked files at all since
they are not subject to ignore patterns.

Make this clearer in the man page.
Reported-by: NGuilherme <guibufolo@gmail.com>
Signed-off-by: NMichael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>