push: require force for refs under refs/tags/

push: require force for refs under refs/tags/

References are allowed to update from one commit-ish to another if the
former is an ancestor of the latter.  This behavior is oriented to
branches which are expected to move with commits.  Tag references are
expected to be static in a repository, though, thus an update to
something under refs/tags/ should be rejected unless the update is
forced.

Signed-off-by: Chris Rorvick <chris@rorvick.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>