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 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.

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