clippy: actually fail if there are any warnings So far, only errors would trip the CI, this makes it so all warnings are elevated to errors, making it a CI fail if any warnings are present. This was the intended behavior, I just missed this when adding clippy.

rust-clippy: fix warning about nested ifs

debugshell: allow commands to be specified as a CLI argument Add a `--command` option to `hg debugshell` that allows the user to pass in Python code to evaluate directly from the command line. This was inspired by the `--command` option present in Facebook's Sapling fork of Mercurial, which in turn was inspired by the `-c` option of the Python interpreter itself. It is particularly useful for writing tests, especially for getting visibility into things that otherwise aren't exposed via debug commands.

heptapod-ci: add `clippy` to the CI This linter mostly makes our code more idiomatic, less surprising, has good suggestions and catches bugs. It's widely used in the Rust community and now part of the default toolchain when using `rustup`.

makefile: add `cargo clippy` to tests if cargo is available This linter mostly makes our code more idiomatic, less surprising, has good suggestions and catches bugs. It's widely used in the Rust community and now part of the default toolchain when using `rustup`.

rust-tests: run `cargo` tests inside the `rust` folder There is no reason to only run inside the `rust/hg-cpython` folder, even if the `cargo test --all` behavior ends up being the same.

rust-clippy: merge "revlog" module definition and struct implementation Module inception is confusing when looking for code and almost never the right choice.