Allow configuring the parent GitHub repository The git integration in build_helper hardcoded `rust-lang/rust` as the parent GitHub repository, and `master` as the branch name. This works great for `rust-lang/rust`, but causes problems in downstream forks like Ferrocene whenever those functions are invoked (like `./x fmt`). In `src/stage0.json` there was already a configuration key for the name of the nightly branch, but it wasn't used by build_helper. This PR adds the `github_repository` key to the file, and requires both values to be passed to build_helper whenever a git function is called. This will allow downstream forks to tweak the values.