qrevert: crash if working tree has tree-conflict

[IWATA Hidetaka]

If working tree has tree-conflict(e.g. parent-loop), qrevert crashes with traceback:
> Traceback (most recent call last):
> File "C:/Program Files/Bazaar/plugins\qbzr\lib\trace.py", line 291, in reports_exception_decorate
> File "C:/Program Files/Bazaar/plugins\qbzr\lib\revert.py", line 167, in initial_load
> File "C:/Program Files/Bazaar/plugins\qbzr\lib\treewidget.py", line 1314, in set_tree
> File "C:/Program Files/Bazaar/plugins\qbzr\lib\treewidget.py", line 422, in set_tree
> UnboundLocalError: local variable 'change' referenced before assignment
>
> bzr 2.0.2 on python 2.5.4 (Windows-XP-5.1.2600-SP3)
> arguments: ['bzr', 'qrevert']
> encoding: 'cp932', fsenc: 'mbcs', lang: None
> plugins:
> qbzr C:\Program Files\Bazaar\plugins\qbzr [0.16.0]

I saw this probrem in qbzr-0.16.0 and trunk(r1103).

[IWATA Hidetaka (hid-iwata)]

To reproduce:

C:\BZR> bzr init A

C:\BZR\A> bzr mkdir foo
C:\BZR\A> bzr mkdir bar
C:\BZR\A> bzr commit -m "initial commit"

C:\BZR> bzr branch A B

C:\BZR\A> bzr move foo bar/foo
C:\BZR\A> bzr commit -m "move foo to bar/foo"

C:\BZR\B> bzr move bar foo/bar
C:\BZR\B> bzr commit -m "move bar to foo/bar"
C:\BZR\B> bzr merge
C:\BZR\B> bzr qrevert