Merge pull request #225 from giuseppe/fuse-overlayfs-naive

overlay: force naiveDiff when using a mount program

drivers/overlay/overlay.go

@@ -340,6 +340,10 @@ func supportsOverlay(home string, homeMagic graphdriver.FsMagic, rootUID, rootGI
 
 func (d *Driver) useNaiveDiff() bool {
 	useNaiveDiffLock.Do(func() {
+		if d.options.mountProgram != "" {
+			useNaiveDiffOnly = true
+			return
+		}
 		if err := doesSupportNativeDiff(d.home, d.options.mountOptions); err != nil {
 			logrus.Warnf("Not using native diff for overlay, this may cause degraded performance for building images: %v", err)
 			useNaiveDiffOnly = true