Microsoft is relying on GitHub and experts in rival programming tools to win back developers. Five years ago, the company was far more insular. When Nadella took over as CEO in 2014, more than a decade of developer defections had left the company in a weak position. Microsoft had been unable to rally engineers to build apps for its phones and tablets. The biggest startups were built on open-source tools, and growing young companies like Uber Technologies Inc. and Airbnb Inc. chose Amazon.com Inc.’s cloud.