This is the "rev-list" analogue to 25fba78 (cat-file:
disable object/refname ambiguity check for batch mode,
2013-07-12). Like cat-file, "rev-list --stdin" may read a
large number of sha1 object names, and the warning check
introduces a significant slow-down.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>