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

commit: don't use hard-coded `.hg/last-message.txt` path in error message Whoever wrote D8463 just used a hard-coded path to `.hg/last-message.text` instead of using the relative path that was already available in the `msgfn` variable (and used just a few lines up in related message). Let's fix that. Differential Revision: https://phab.mercurial-scm.org/D12585