update-ref: fail create operation over stdin if ref already exists

Signed-off-by: Aman Gupta <aman@tmm1.net>
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Acked-by: Brad King <brad.king@kitware.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/update-ref.c

@@ -147,6 +147,7 @@ static void parse_cmd_create(const char *next)
 	struct ref_update *update;
 
 	update = update_alloc();
+	update->have_old = 1;
 
 	if ((next = parse_first_arg(next, &ref)) != NULL && ref.buf[0])
 		update_store_ref_name(update, ref.buf);

t/t1400-update-ref.sh

@@ -820,7 +820,18 @@ test_expect_success 'stdin -z update ref fails with bad old value' '
 	test_must_fail git rev-parse --verify -q $c
 '
 
+test_expect_success 'stdin -z create ref fails when ref exists' '
+	git update-ref $c $m &&
+	git rev-parse "$c" >expect &&
+	printf $F "create $c" "$m~1" >stdin &&
+	test_must_fail git update-ref -z --stdin <stdin 2>err &&
+	grep "fatal: Cannot lock the ref '"'"'$c'"'"'" err &&
+	git rev-parse "$c" >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'stdin -z create ref fails with bad new value' '
+	git update-ref -d "$c" &&
 	printf $F "create $c" "does-not-exist" >stdin &&
 	test_must_fail git update-ref -z --stdin <stdin 2>err &&
 	grep "fatal: invalid new value for ref $c: does-not-exist" err &&