rhg-cat: fallback when detecting `.` or `..` path segments
We do not normalize paths correctly yet, so exclude the shortcuts.
Differential Revision: https://phab.mercurial-scm.org/D11378
rhg: add support for calling `rhg cat` without a revision
Turns out the necessary pieces were there already.
Like the Python implementation, we default to the first parent of the dirstate.
Differential Revision: https://phab.mercurial-scm.org/D11377