vfs: stop change notify polling clearing so much of the directory cache

Before this change, change notify polls would clear the directory
cache recursively. So uploading a file to the root would clear the
entire directory cache.

After this change we just invalidate the directory cache of the parent
directory of the item and if the item was a directory we invalidate it
too.

vfs/vfs.go

@@ -232,7 +232,7 @@ func New(f fs.Fs, opt *Options) *VFS {
 	// Start polling function
 	if do := vfs.f.Features().ChangeNotify; do != nil {
 		vfs.pollChan = make(chan time.Duration)
-		do(context.TODO(), vfs.root.ForgetPath, vfs.pollChan)
+		do(context.TODO(), vfs.root.changeNotify, vfs.pollChan)
 		vfs.pollChan <- vfs.Opt.PollInterval
 	} else {
 		fs.Infof(f, "poll-interval is not supported by this remote")