cat-file: stop returning value from batch_one_object

cat-file: stop returning value from batch_one_object

If batch_one_object returns an error code, we stop reading
input.  However, it will only do so if we feed it NULL,
which cannot happen; we give it the "buf" member of a
strbuf, which is always non-NULL.

We did originally stop on other errors (like a missing
object), but this was changed in 3c076db (cat-file --batch /
--batch-check: do not exit if hashes are missing,
2008-06-09). These days we keep going for any per-object
error (and print "missing" when necessary).

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