BRUSSELS/ATHENS (Reuters) – Greece’s leftwing Prime Minister Alexis Tsipras faces a showdown with rebels in his own party on Tuesday furious at his capitulation to German demands for one of the most sweeping austerity packages ever demanded of a euro zone government. Just hours after a deal that saw Greece surrender much of its sovereignty to outside supervision in return for agreeing to talks on an 86 billion euro bailout, doubts were already emerging about whether Tsipras would be able to hold his government together.