Add .keep files for empty directories and ignore them in tests

Ignore those .keep file which are to keep an empty generated directory alive
to compare with newly generated branch.

codelab/codelab_test.go

@@ -181,6 +181,11 @@ func compareAll(t *testing.T, original, generated string, ignoresf []string) {
 		relp = relp[1:]
 		p := path.Join(generated, relp)
 
+		// .keep files are only for keeping directory creations in remote git repo
+		if filepath.Base(p) == ".keep" {
+			return nil
+		}
+
 		fo, err := os.Stat(p)
 		if err != nil {
 			t.Fatalf("%s doesn't exist while %s does", p, f)
@@ -230,6 +235,11 @@ func compareAll(t *testing.T, original, generated string, ignoresf []string) {
 		relp = relp[1:]
 		p := path.Join(original, relp)
 
+		// .keep files are only for keeping directory creations in remote git repo
+		if filepath.Base(p) == ".keep" {
+			return nil
+		}
+
 		if _, err := os.Stat(p); err != nil {
 			difff = append(difff, relp)
 			if !testtools.StringContains(ignoresf, relp) {