vfs: add --vfs-refresh flag to read all the directories on start

Refreshes the directory listing recursively at VFS start time.

vfs/vfs.go

@@ -232,12 +232,26 @@ func New(f fs.Fs, opt *vfscommon.Options) *VFS {
 	// removed when the vfs is finalized
 	cache.PinUntilFinalized(f, vfs)
 
+	// Refresh the dircache if required
+	if vfs.Opt.Refresh {
+		go vfs.refresh()
+	}
+
 	// This can take some time so do it after the Pin
 	vfs.SetCacheMode(vfs.Opt.CacheMode)
 
 	return vfs
 }
 
+// refresh the directory cache for all directories
+func (vfs *VFS) refresh() {
+	fs.Debugf(vfs.f, "Refreshing VFS directory cache")
+	err := vfs.root.readDirTree()
+	if err != nil {
+		fs.Errorf(vfs.f, "Error refreshing VFS directory cache: %v", err)
+	}
+}
+
 // Stats returns info about the VFS
 func (vfs *VFS) Stats() (out rc.Params) {
 	out = make(rc.Params)