setup: fix mac build broken by e42d18538e1d Sometimes xcodebuild prints warnings to stderr, but runcmd() assumes anything printed to stderr implies failure. Since runcmd() was originally only intended to run hg, this was fine until it was pressed into service for running xcodebuild. Thus: split runcmd() into two parts: runcmd(), which does the minimal amount of work to run a subprocess, and runhg(), which calls runcmd().