SAVANNAH — Savannah fell to British troops in the American Revolution and surrendered to the Union Army during the Civil War. Now Georgia's oldest city is preparing to retreat from a law that for decades has required tour guides to prove to City Hall that they know history. The Savannah City Council is scheduled to vote Thursday on a proposed repeal of tour guide regulations — in place since 1978 — that require all guides to earn a city license by passing a history test before they can take paying customers sightseeing.