upload-archive: convert sprintf to strbuf
When we report an error to the client, we format it into a
fixed-size buffer using vsprintf(). This can't actually
overflow in practice, since we only format a very tame
subset of strings (mostly strerror() output). However, it's
hard to tell immediately, so let's just use a strbuf so
readers do not have to wonder.
We do add an allocation here, but the performance is not
important; the next step is to call die() anyway.
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>