BERLIN—Germany is extending its lockdown until April 18 and calling on citizens to stay at home for five days over the Easter holidays to try to break a third wave of the COVID-19 pandemic, Chancellor Angela Merkel said early on Tuesday. In talks that ran deep into the night, Merkel pushed the leaders of Germany’s 16 states to take a tougher stance to fight the pandemic, reversing plans for a gradual re-opening of the economy agreed earlier this month after a sharp rise in the infection rate. “We are now basically in a new pandemic. The British mutation has become dominant,” Merkel told a news conference. “Fundamentally, we face a new virus of the same kind but with very different characteristics. More deadly, more infectious, and infectious for longer.” Germany started cautiously easing restrictions earlier this month. But the spread of more infectious variants of the virus has pushed up …