diff: Fix overlapping filenames

The filename can overlap when multiple resources have the same name (but
obviously are of a different type). Include the name of the type in the
file name to prevent the overlap.

hack/testdata/diff/configmap.yaml

@@ -0,0 +1,4 @@
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: test

hack/testdata/diff/deployment.yaml

@@ -0,0 +1,17 @@
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: test
+spec:
+  replicas: 3
+  selector:
+    matchLabels:
+      name: test
+  template:
+    metadata:
+      labels:
+        name: test
+    spec:
+      containers:
+      - name: nginx
+        image: nginx

hack/testdata/diff/pod.yaml

@@ -0,0 +1,8 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: test
+spec:
+  containers:
+  - name: nginx
+    image: nginx

hack/testdata/diff/secret.yaml

@@ -0,0 +1,4 @@
+apiVersion: v1
+kind: Secret
+metadata:
+  name: test