DETROIT—General Motors posted a healthy $2.8 billion second-quarter profit Wednesday, but its cautious outlook for the rest of the year spooked investors. The Detroit automaker made the strong profit despite a global shortage of computer chips that have forced it and other companies to temporarily close factories. While GM raised its outlook for the year, it also cautioned about high commodity prices and lower production due to the chip shortage, and it warned that the highly contagious delta variant of the coronavirus could hamper its supply chain. The cautious tone rattled investors, who pushed GM shares down more than 9 percent an hour before Wednesday’s closing bell. Chief Financial Officer Paul Jacobson predicted that GM would produce 100,000 fewer vehicles in the second half of the year compared with the first, even as other companies forecast improvements in semiconductor production. GM said it managed the shortage well and was able …