When Kilmarnock travelled to Parkhead on Saturday, few would have imagined – even with Celtic out of sorts and a pale imitation of their Ange Postecoglou-era pomp – that the home goalkeeper would find himself with a lot more on his plate than the visiting glovesman Will Dennis. By the time the full-time whistle echoed around a half-empty stadium – a chorus of boos audible despite a sea of empty green seats – Joe Hart had been kept far bisier than his opposite number, Celtic’s veteran stopper denying an impressive Killie side on a number of occasions before David Watson’s deserved extra-time eq…