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 commit
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.
This commit allows users to change the contents of the "config" key in
src/stage0.json without having it overridden the next time the
bump-stage0 tool is executed.
It looks like the last time had left some remaining cfg's -- which made me think
that the stage0 bump was actually successful. This brings us to a released 1.62
beta though.
