By Nell Mackenzie LONDON (Reuters) – In the first half of September bets against sterling posted the biggest two-week rise since 2013, a sign that hedge funds may have ramped up short positions just as Liz Truss became Britain’s new Prime Minister. According to Vanda Research, which uses data from the U.S. Commodity Futures Trading Commission (CFTC), short contracts of GBP/USD made by leveraged funds in the first two weeks of September jumped 17 percentage points. Sterling hit a record low on Monday against the dollar, and British bond prices collapsed as fears mounted over the new government’…