do not pretend sha1write returns errors

do not pretend sha1write returns errors

The sha1write function returns an int, but it will always be
"0". The failure-prone parts of the function happen in the
"flush" callback, which cannot pass an error back to us. So
we just end up calling die() during the flush.

Let's just drop the return value altogether, as it only
confuses callers into thinking that it might be useful.

Only one call site actually checked the return value. We can
drop that check, since it just led to a die() anyway.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>