Fix "git-fetch --tags" exit status when nothing has been changed

After commit 55b7835e1b81a6debc7648149d2b8a4c5c64ddba git-fetch --tags
exits with status 1 when no tags have been changed, which breaks calling
git-fetch from scripts.

Signed-off-by: Sergey Vlasov <vsu@altlinux.ru>
Signed-off-by: Junio C Hamano <junkio@cox.net>