Fix selection of kustomization resource from multi doc yaml

Signed-off-by: Marcus Weiner <marcus.weiner@gmail.com>

cmd/flux/diff_kustomization_test.go

@@ -91,6 +91,12 @@ func TestDiffKustomization(t *testing.T) {
 			objectFile: "./testdata/diff-kustomization/stringdata-sops-secret.yaml",
 			assert:     assertGoldenFile("./testdata/diff-kustomization/diff-with-drifted-stringdata-sops-secret.golden"),
 		},
+		{
+			name:       "diff where kustomization file has multiple objects with the same name",
+			args:       "diff kustomization podinfo --path ./testdata/build-kustomization/podinfo --progress-bar=false --kustomization-file ./testdata/diff-kustomization/flux-kustomization-multiobj.yaml",
+			objectFile: "",
+			assert:     assertGoldenFile("./testdata/diff-kustomization/nothing-is-deployed.golden"),
+		},
 	}
 
 	tmpl := map[string]string{

cmd/flux/testdata/diff-kustomization/flux-kustomization-multiobj.yaml

@@ -0,0 +1,19 @@
+apiVersion: v1
+kind: Namespace
+metadata:
+  name: podinfo
+
+---
+apiVersion: kustomize.toolkit.fluxcd.io/v1
+kind: Kustomization
+metadata:
+  name: podinfo
+spec:
+  interval: 5m0s
+  path: ./kustomize
+  force: true
+  prune: true
+  sourceRef:
+    kind: GitRepository
+    name: podinfo
+  targetNamespace: default