config: track "source" along side value Currently the source is stored in a entirely different way than the data. This is impractical. Especially if we are about to add more of such metadata. So lets put them back together. Differential Revision: https://phab.mercurial-scm.org/D9925

config: use a new `alter` method in `fixconfig` The `set` function is doing various work related to metadata (eg: the source, later the level). However the `fixconfig` call only updating some values according to standard processing, not changing any of the related metadata. So we introduce a new method and use it there. Differential Revision: https://phab.mercurial-scm.org/D9924

copies-rust: remove an unnecessary format!() inside assert!() The `assert!()` macro supports formatting. Rust 2021 no longer supports an unnecessary `format!()` inside it. I noticed because I recently update my Rust toolchain and `test-check-cargo-lock.t` started failing. Differential Revision: https://phab.mercurial-scm.org/D10085