Add kpod diff command

kpod diff reports on differences between two layers, specified as
layer IDs, containers, or images.  In the case of containers or
images, kpod diff produces a diff for the top layer

Signed-off-by: Ryan Cole <rcyoalne@gmail.com>

test/kpod_diff.bats

@@ -0,0 +1,37 @@
+#/usr/bin/env bats
+
+load helpers
+
+IMAGE="alpine:latest"
+ROOT="$TESTDIR/crio"
+RUNROOT="$TESTDIR/crio-run"
+KPOD_OPTIONS="--root $ROOT --runroot $RUNROOT --storage-driver vfs"
+
+function teardown() {
+    cleanup_test
+}
+
+@test "test diff of image and parent" {
+    run ${KPOD_BINARY} $KPOD_OPTIONS pull $IMAGE
+    [ "$status" -eq 0 ]
+    run ${KPOD_BINARY} $KPOD_OPTIONS diff $IMAGE
+    [ "$status" -eq 0 ]
+    echo "$output"
+    run ${KKPOD_BINARY} $KPOD_OPTIONS rmi $IMAGE
+}
+
+@test "test diff on non-existent layer" {
+    run ${KPOD_BINARY} $KPOD_OPTIONS diff "abc123"
+    [ "$status" -ne 0 ]
+    echo "$output"
+}
+
+@test "test diff with json output" {
+    run ${KPOD_BINARY} $KPOD_OPTIONS pull $IMAGE
+    [ "$status" -eq 0 ]
+    # run bash -c "${KPOD_BINARY} ${KPOD_OPTIONS} diff --json $IMAGE | python -m json.tool"
+    run ${KPOD_BINARY} $KPOD_OPTIONS diff --json $IMAGE
+    [ "$status" -eq 0 ]
+    echo "$output"
+    run ${KKPOD_BINARY} $KPOD_OPTIONS rmi $IMAGE
+}