The filter istream pulls data from an "upstream" stream,
running it through a filter function. However, we did not
properly notice when the upstream filter yielded an error,
and just returned what we had read. Instead, we should
propagate the error.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>