RFC: merge and revert - better diagnostic messages

[Jari Aalto]

Mistakes happen:

        bzr merge that/feature
        <oops, not that!>
        bzr revert
        Revert phase: ..............
        
But this did not revert the merge as I inspected the "bzr log" and
the file that was affected by merge. But the command seemes obvious
at the time:

   - one did a merge
   - a revert would certainly revert the last change, no? No. Hm.

SUGGESTION

It would be good if "bzr revert" command reported something if there
was nothing to revert. It could even report:

  Nothing to do. If you meant to revert a merge, please consult "bzr
  help revert"
        
Jari