rust-index: synchronize append method We now append to the Rust index just as we do to the C index. Future steps will bring the two indexes further together until we can rip the C index entirely when running Rust code.

rust-revlog: teach the revlog opening code to read the repo options This will become necessary as we start writing revlog data from Rust.

rust-index: pass data down to the Rust index This will allow us to start keeping the Rust index synchronized with the cindex as we gradually implement more and more methods in Rust. This will eventually be removed.