Add diff service implementation

Add snapshot subcommand to ctr for creating diffs of RW layers.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

cmd/ctr/snapshot.go

@@ -0,0 +1,50 @@
+package main
+
+import (
+	"context"
+	"errors"
+	"fmt"
+
+	"github.com/containerd/containerd/rootfs"
+	"github.com/urfave/cli"
+)
+
+var snapshotCommand = cli.Command{
+	Name:      "snapshot",
+	Usage:     "snapshot a container into an archive",
+	ArgsUsage: "",
+	Flags: []cli.Flag{
+		cli.StringFlag{
+			Name:  "id",
+			Usage: "id of the container",
+		},
+	},
+	Action: func(clicontext *cli.Context) error {
+		id := clicontext.String("id")
+		if id == "" {
+			return errors.New("container id must be provided")
+		}
+
+		snapshotter, err := getSnapshotter(clicontext)
+		if err != nil {
+			return err
+		}
+
+		differ, err := getDiffService(clicontext)
+		if err != nil {
+			return err
+		}
+
+		contentRef := fmt.Sprintf("diff-%s", id)
+
+		d, err := rootfs.Diff(context.TODO(), id, contentRef, snapshotter, differ)
+		if err != nil {
+			return err
+		}
+
+		// TODO: Track progress
+		fmt.Printf("%s %s\n", d.MediaType, d.Digest)
+
+		return nil
+	},
+}