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