rust-index: expose a method to retrieve the C index The code for `shortesthexnodeidprefix` need to access the actual C index. For now we grant its wish and expose a method to do so. Once we have the nodemap in Rust, we will be able to implement the same feature from rust and we will be able to drop this method. Differential Revision: https://phab.mercurial-scm.org/D7658