Donald Trump granted a presidential pardon on Friday to a former New York police sergeant who was convicted of helping China try to scare an ex-official into going back to his homeland, a prominent case in US authorities’ efforts to combat what they claim are Beijing’s far-flung efforts to repress critics. Michael McMahon was sentenced this spring to 18 months in prison for his role in what a federal judge called “a campaign of transnational repression”. He insisted he was innocent, saying he was “unwittingly used” when he took what…