helper/test-ref-store: fix "new-sha1" vs "old-sha1" typo

It looks like it is a copy-paste error  made in 80f2a6097c
(t/helper: add test-ref-store to test ref-store functions,
2017-03-26) to pass "old-sha1" instead of "new-sha1" to
notnull() when we get the new sha1 argument from
const char **argv.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Acked-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/helper/test-ref-store.c

@@ -211,7 +211,7 @@ static int cmd_update_ref(struct ref_store *refs, const char **argv)
 {
 	const char *msg = notnull(*argv++, "msg");
 	const char *refname = notnull(*argv++, "refname");
-	const char *new_sha1_buf = notnull(*argv++, "old-sha1");
+	const char *new_sha1_buf = notnull(*argv++, "new-sha1");
 	const char *old_sha1_buf = notnull(*argv++, "old-sha1");
 	unsigned int flags = arg_flags(*argv++, "flags");
 	unsigned char old_sha1[20];