qrevert shows TREE_ROOT

[Alexander Belchenko]

Running qrevert in the branch without commits shows TREE_ROOT entry as '/'. Actually user can't revert this entry, and running qrevert with this entry checked leads to error. To reproduce:

bzr init .
bzr qrevert

(see attached image)

Clicking OK produce traceback:

Traceback (most recent call last):
  File "C:\work\Bazaar\plugins\qbzr\lib\subprocess.py", line 144, in accept
    self.start()
  File "C:\work\Bazaar\plugins\qbzr\lib\revert.py", line 108, in start
    self.process_widget.start(self.tree.basedir, *args)
  File "C:\work\Bazaar\plugins\qbzr\lib\subprocess.py", line 329, in start
    self.start_multi(((dir, args),))
  File "C:\work\Bazaar\plugins\qbzr\lib\subprocess.py", line 335, in start_multi
    self._start_next()
  File "C:\work\Bazaar\plugins\qbzr\lib\subprocess.py", line 339, in _start_next
    args = ' '.join('"%s"' % a.replace('"', '\\"') for a in args)
  File "C:\work\Bazaar\plugins\qbzr\lib\subprocess.py", line 339, in <genexpr>
    args = ' '.join('"%s"' % a.replace('"', '\\"') for a in args)
AttributeError: 'NoneType' object has no attribute 'replace'