rhg: make it possible to opt out of `rhg cat` The reason an opt-out is needed is that there are still behavior differences between `rhg cat` and `hg cat`: - it does not interpret relative paths correctly - it does not interpret patterns correctly, e.g. 're:foobar$' would be interpreted as a verbatim filename - it does not implement the correct semantics of relpath matcher: if given a directory, `hg` concatenates all files in this directory, while `rhg` simply complains Differential Revision: https://phab.mercurial-scm.org/D11723