By Lawrence Delevingne and Amanda Cooper (Reuters) -U.S. and global equities staged a mild comeback on Wednesday as the Bank of England said it would step into the bond market to stem a damaging rise in borrowing costs, an attempt to dampen investors fears of contagion across the financial system. The BoE said it would temporarily buy long-dated bonds – linked most closely to workers’ pensions and home loans – in light of a surge in 30-year UK bond yields above 5%, their highest since 2002. Sterling, which hit record lows against the dollar on Monday, was last down 0.1%, whipsawed in volatile …