Do not allow fast-forwarding of references that point to a tag object.
Updating from a tag is potentially destructive since it would likely
leave the tag dangling. Disallowing updates to a tag also makes sense
semantically and is consistent with the behavior of lightweight tags.
Signed-off-by: Chris Rorvick <chris@rorvick.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>