rev-list: disable object/refname ambiguity check with --stdin

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>

revision.c

@@ -1541,6 +1541,10 @@ static void read_revisions_from_stdin(struct rev_info *revs,
 {
 	struct strbuf sb;
 	int seen_dashdash = 0;
+	int save_warning;
+
+	save_warning = warn_on_object_refname_ambiguity;
+	warn_on_object_refname_ambiguity = 0;
 
 	strbuf_init(&sb, 1000);
 	while (strbuf_getwholeline(&sb, stdin, '\n') != EOF) {
@@ -1562,7 +1566,9 @@ static void read_revisions_from_stdin(struct rev_info *revs,
 	}
 	if (seen_dashdash)
 		read_pathspec_from_stdin(revs, &sb, prune);
+
 	strbuf_release(&sb);
+	warn_on_object_refname_ambiguity = save_warning;
 }
 
 static void add_grep(struct rev_info *revs, const char *ptn, enum grep_pat_token what)