check-ignore: add -n / --non-matching option

check-ignore: add -n / --non-matching option

If `-n` or `--non-matching` are specified, non-matching pathnames will
also be output, in which case all fields in each output record except
for <pathname> will be empty.  This can be useful when running
check-ignore as a background process, so that files can be
incrementally streamed to STDIN, and for each of these files, STDOUT
will indicate whether that file matched a pattern or not.  (Without
this option, it would be impossible to tell whether the absence of
output for a given file meant that it didn't match any pattern, or
that the result simply hadn't been flushed to STDOUT yet.)

Signed-off-by: Adam Spiers <git@adamspiers.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>