StorageDriver: GCS: remove support for directory Moves

The Move operation is only used to move uploaded blobs
to their final destination. There is no point in implementing
Move on "folders". Apart from simplifying the code, this also
saves an HTTP request.

Signed-off-by: Arthur Baars <arthur@semmle.com>

docs/storage/driver/gcs/gcs_test.go

@@ -175,3 +175,40 @@ func TestEmptyRootList(t *testing.T) {
 		}
 	}
 }
+
+// TestMoveDirectory checks that moving a directory returns an error.
+func TestMoveDirectory(t *testing.T) {
+	if skipGCS() != "" {
+		t.Skip(skipGCS())
+	}
+
+	validRoot, err := ioutil.TempDir("", "driver-")
+	if err != nil {
+		t.Fatalf("unexpected error creating temporary directory: %v", err)
+	}
+	defer os.Remove(validRoot)
+
+	driver, err := gcsDriverConstructor(validRoot)
+	if err != nil {
+		t.Fatalf("unexpected error creating rooted driver: %v", err)
+	}
+
+	ctx := ctx.Background()
+	contents := []byte("contents")
+	// Create a regular file.
+	err = driver.PutContent(ctx, "/parent/dir/foo", contents)
+	if err != nil {
+		t.Fatalf("unexpected error creating content: %v", err)
+	}
+	defer func() {
+		err := driver.Delete(ctx, "/parent")
+		if err != nil {
+			t.Fatalf("failed to remove /parent due to %v\n", err)
+		}
+	}()
+
+	err = driver.Move(ctx, "/parent/dir", "/parent/other")
+	if err == nil {
+		t.Fatalf("Moving directory /parent/dir /parent/other should have return a non-nil error\n")
+	}
+}