- Free a mergelist's parent_dirscanned filters the last time it is
  popped or as soon as the filters are discarded due to the "n"
  modifier.  Aside from not leaking memory, this is needed to clean up
  any mergelists defined during the parent_dirscan to avoid crashing by
  trying to restore nonexistent state for them in pop_local_filters.
- Make push_local_filters save the current mergelist_cnt, and make
  pop_local_filters assert that it has the correct number of mergelists
  before restoring their state.
- Assert that mergelists get deactivated in strict LIFO order to catch
  any glitches as soon as they happen.  Free linked lists of filters in
  reverse order to make that the case.
- Add a bunch of mergelist-related debug output (--debug=filter2).