Ever since Tesla’s Model S went on sale in 2012, the start-up automaker has had the market for high-end electric cars to itself. But that is changing as other automakers start to introduce their own models. On Monday, at the Frankfurt Auto Show in Germany, Porsche unveiled its all-electric Mission E concept car. A four-door, four-seat luxury performance sedan with futuristic 911 design cues, Mission E will be the first all-electric model Porsche will offer…