(Reuters) - Gains for oil company shares pushed London's FTSE 100 into the black on Tuesday, helped by a prediction that OPEC would supply a shrinking portion of global supply, while Primark owner Associated British Foods jumped 5% after strong results. The FTSE 100 gained 0.2% by 0810 GMT, hovering close to a one-month high it touched in the previous session, while the mid-cap index was 0.1% higher. Oil majors Shell and BP , which fell last week after reporting falls in profit, were the..