Add support for checking current state without changing anything

[chevdor]

This could be in the form of a new check command or based on update with a new --dry-run flag.
The goal is to be able to check whether a project is up to date without having to touch the .lock file.

[bkchr]

The goal is to be able to check whether a project is up to date without having to touch the .lock file.

Up to date in what sense?

[chevdor]

To check if diener would modify any Cargo.toml

[bkchr]

In what context do you need this?

[chevdor]

Ensuring in the release process (CI) that the diener run was not omiited and was run with the right args.