(Reuters) - UK shares rebounded on Tuesday in the absence of any immediate escalation in the Middle East tensions, while a strong Christmas performance update from supermarket group Morrisons lifted the sector. The FTSE 100 was up 0.2% by 0809 GMT, with gains led by supermarket groups Morrisons and Sainsbury's . The killing of a top Iranian military commander by the United States had sparked a selloff on Monday, with both the UK benchmark indexes losing nearly 1%. Midcaps added 0.4% as..