set_stacked_on_location leaves a locked repossitory with an unlocked fallback repository

This may be related to 283457, or at least easiest to fix at the same time:

mwh@grond:repro$ bzr init --1.6 a
Standalone tree (format: 1.6)
Location:
  branch root: a
mwh@grond:repro$ bzr init --1.6 b
Standalone tree (format: 1.6)
Location:
  branch root: b
mwh@grond:repro$ cd a
mwh@grond:a$ python2.5
Python 2.5.2 (r252:60911, Jul 31 2008, 17:31:22)
[GCC 4.2.3 (Ubuntu 4.2.3-2ubuntu7)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from bzrlib import branch
>>> b = branch.Branch.open('.')
>>> b.lock_write()
'982x9skvm333rkpyrzku'
>>> b.set_stacked_on_url('../b')
>>> b.unlock()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.5/site-packages/bzrlib/branch.py", line 1519, in unlock
    self.repository.unlock()
  File "/usr/lib/python2.5/site-packages/bzrlib/repofmt/pack_repo.py", line 1876, in unlock
    repo.unlock()
  File "/usr/lib/python2.5/site-packages/bzrlib/repofmt/pack_repo.py", line 1878, in unlock
    self.control_files.unlock()
  File "/usr/lib/python2.5/site-packages/bzrlib/lockable_files.py", line 282, in unlock
    raise errors.LockNotHeld(self)
bzrlib.errors.LockNotHeld: Lock not held: LockableFiles(lock, file:///home/mwh/tmp/repro/b/.bzr/repository/)
>>>