When forcing to overwrite an annotated tag, there are good chances one
wants to keep the old annotation, or modify it, not start from scratch.

This is obviously only triggered for annotated tagging (-a or -s).
Signed-off-by: NMike Hommey <mh@glandium.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>