rust-tests: add test to check if `Cargo.lock` is up-to-date An out-of-date `Cargo.lock` is the source of rebase conflicts, prevents a reproductible build, introduces desync between the series that introduces a change in dependencies and the one that eventually commits the `Cargo.lock`, and is just a general annoyance. This commit demonstrates that the test works, the next one fixes the currently out-of-date `Cargo.lock`. Differential Revision: https://phab.mercurial-scm.org/D9079

scmutil: introduce function to check whether repo uses treemanifest or not In an upcoming patch, I wanted to check whether current repo uses treemanifest or not. I looked for a function and found that at all places we manually check for the requirement in repo requirements. I guess having a dedicated function for that is much better. Differential Revision: https://phab.mercurial-scm.org/D8981