rhg: make it possible to opt out of `rhg cat`

Authored by aalekseyev.


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