LONDON (Reuters) - Asda, the British supermarket arm of U.S. retail giant Walmart , said uncertainty surrounding Brexit was affecting its customers as it eked out a small rise in second quarter sales, helped by the later timing of Easter this year.Britain's new prime minister Boris Johnson has promised to take Britain out of the European Union by Oct. 31, with or without a deal, setting the scene for a showdown in parliament where lawmakers are opposed to a divorce without a transition..