THE FEROCITY with which many Gibraltarians will denounce the deal expected to be announced today between Britain and Spain to share sovereignty over the Rock may be a moment of catharsis. It reflects the understandable feeling of betrayal shared by many other communities left behind by the receding of the red ink from the map of the world. The force of the Gibraltarians' insistence on their Britishness carries echoes of the excessive loyalty to symbols of Britishness demonstrated by the unionists in Northern Ireland.