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
$ LANG=nonexistent LC_ALL=nonexistent LANGUAGE=nonexistent hg version -q
Mercurial Distributed SCM (version *) (glob)