initial josh subtree sync This demonstrates what a josh-based rustup would look like with my patched josh. To create it I did ``` git fetch http://localhost:8000/rust-lang/rust.git:start=75dd959a3a40eb5b4574f8d2e23aa6efbeb33573[:prefix=src/tools/miri]:/src/tools/miri.git master git merge FETCH_HEAD ./rustup-toolchain HEAD && ./miri fmt git commit -am rustup ``` Unlike the [previous attempt](https://github.com/rust-lang/miri/pull/2554), this does not add a new root commit to the repo. Once we merge this, we committed to using josh for subtree syncing, and in particular a version of josh that includes https://github.com/josh-project/josh/pull/961 (or something compatible).