ref-filter: handle nested tags in --points-at option

ref-filter: handle nested tags in --points-at option

Tags are dereferenced until reaching a different object type to handle
nested tags, e.g. on checkout. In contrast, "git tag --points-at=..."
fails to list such nested tags because only one level of indirection is
obtained in filter_refs(). Implement the recursive dereferencing for the
"--points-at" option when filtering refs to unify the behaviour.

Signed-off-by: Jan Klötzke <jan@kloetzke.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>