By Yadarisa Shabong and Chandini Monnappa (Reuters) -Former Barclays veterans Bob Diamond and Rich Ricci have agreed an all-share merger of Panmure Gordon and UK rival Liberum, the firms said on Tuesday, creating Britain’s largest independent investment bank amid an extended dealmaking slump. The merger is intended to steer the enlarged group through challenging times for Europe’s investment banks, with business hurt by rising geopolitical tensions and interest rate uncertainty. It comes hot on the heels of Cenkos Securities and finnCap’s merger to create Cavendish, as well as Redburn and Gene…