upload-pack: propagate return value from object filter config callback

upload-pack: propagate return value from object filter config callback

If we encounter an error in parse_filter_object_config(), we'll complain
to stderr but won't actually propagate the return value up the stack.
This is unlike most of our config callbacks, which return the error to
git_config() so it can die (this includes the call just below us to
parse_hide_refs_config(), which can also produce errors).

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