Add pipeline requirement for current-patchset.

Zuul can get itself into test loops when it receives comments for old
patchsets because it attempts to vote on old patchsets which is not
possible so the vote based requeuing stuff falls over and loops. Add
an option to ignore old patchsets.

Co-Authored-By: James E. Blair <jeblair@openstack.org>
Change-Id: Ie3cd82fb9535e27b549a2483ac4fa9736930b044