blackmatch fuse doesn't know what to think about immutable directories

[zooko]

As reported by François over on comment:5:ticket:830:

Sat Jan  9 22:25:05 2010: exception:
Traceback (most recent call last):
  File "/home/francois/WORK/dev/tahoe/contrib/fuse/impl_c/blackmatch.py", line 177, in inner_logexc
    ret = meth(self, *args, **kwargs)
  File "/home/francois/WORK/dev/tahoe/contrib/fuse/impl_c/blackmatch.py", line 314, in done
    precondition(not self.read_heap, self.read_heap, self.target_size, self.get_size())
  File "/usr/local/lib/python2.6/dist-packages/allmydata_tahoe-1.5.0_r4054-py2.6.egg/allmydata/util/assertutil.py", line 46, in precondition
    raise AssertionError, "".join(msgbuf)
AssertionError: precondition: [(3832, 0, <Deferred at 0x3c94440>)] <type 'list'>, 264 <type 'int'>, 264 <type 'int'>