Making errbuf an optional argument complicates error reporting. Fix this
by making all callers supply an errbuf, even if they may ignore it. This
will be important in follow-up patches where the filter-spec parsing has
more pitfalls and possible errors.
Signed-off-by: Matthew DeVore <matvore@google.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>