fix the qt 4.5 -> 4.6 update issue.

fix the qt 4.5 -> 4.6 update issue.
since we usually deal with directories in a global way, replacing a
directory with something else didn't work.
the fix is to record removed directories in Vstat in a "lazy" way:
we only test these objects for existence if we add something else at
the same location. Since this happens rarely, we can afford to perform
the whole SharedItems dance twice...