rhg: use `Command::exec` instead of `Command::status` `rhg` is supposed to be a transparent executable, using a subprocess defeats that purpose. See inline comments for more details. This also introduces the `which` crate to check if the fallback executable actually exists to help debugging (plain `execve` doesn't give much information). The error code 253 is used to signify that the fallback is not found, but may mean in the future that it is otherwise invalid if we start being more specific. Differential Revision: https://phab.mercurial-scm.org/D12578

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