CAIRO — Egypt’s constitution is being rewritten by a small group appointed by the military-backed government, overhauling the previous, Islamist-driven charter. This will be the country’s fourth constitution in less than three years. A 50-member committee is overseeing what’s meant to be the first step of a “roadmap” to reshape Egypt’s government following the July ousting of the country’s first freely-elected president, Islamist Mohamed Morsi.  Morsi pushed through last year’s charter, written mainly by Islamists, using extraordinary powers.