Optimize a decAutoUpdate+incAutoUpdate code path which led to unregistering+reregistering to KDirWatch for the same path,
which is unnecessary, and probaby caused the bug Lubos reported, where OOo's KFileDialog wouldn't show files created
since the last filedialog, because that event gets processed very late (no event loop while no dialog is up), after the
deregister+reregister.

Tested with "./kdirlistertest testOpenUrl testOpenUrlFromCache" and watching the KDirWatch debug output.