Often our short summaries will pick up a Bors "Auto merge of #xxxx ...`
commit message. Replace these with something like `rust-lang/rust#1234`
to avoid broken links when going between repositories.
Create a crate that handles pulling from and pushing to rust-lang/rust.
This can be invoked with the following:
$ cargo run -p josh-sync -- rustc-pull
$ RUSTC_GIT=/path/to/rust/checkout cargo run -p josh-sync -- rustc-push <username>