Dan Ammann, the former president of General Motors, has been appointed head of Exxon’s oil and gas business, after serving as head of its low-carbon solutions for two years. The appointment of Ammann follows the retirement of the current president of the Upstream division, Liam M. Mallon, which will take effect at the start of February. “These appointments continue the company’s approach of leveraging the diverse skills and experiences of a very accomplished executive team to bring fresh perspectives and approaches to each of…