Refactor differ into separate package

Add differ options and package with interface.
Update optional values on diff interface to use options.

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

diff/diff.go

@@ -0,0 +1,69 @@
+package diff
+
+import (
+	"github.com/containerd/containerd/mount"
+	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
+	"golang.org/x/net/context"
+)
+
+// Config is used to hold parameters needed for a diff operation
+type Config struct {
+	// MediaType is the type of diff to generate
+	// Default depends on the differ,
+	// i.e. application/vnd.oci.image.layer.v1.tar+gzip
+	MediaType string
+
+	// Reference is the content upload reference
+	// Default will use a random reference string
+	Reference string
+
+	// Labels are the labels to apply to the generated content
+	Labels map[string]string
+}
+
+// Opt is used to configure a diff operation
+type Opt func(*Config) error
+
+// Differ allows the apply and creation of filesystem diffs between mounts
+type Differ interface {
+	// Apply applies the content referred to by the given descriptor to
+	// the provided mount. The method of applying is based on the
+	// implementation and content descriptor. For example, in the common
+	// case the descriptor is a file system difference in tar format,
+	// that tar would be applied on top of the mounts.
+	Apply(ctx context.Context, desc ocispec.Descriptor, mount []mount.Mount) (ocispec.Descriptor, error)
+
+	// DiffMounts computes the difference between two mounts and returns a
+	// descriptor for the computed diff. The options can provide
+	// a ref which can be used to track the content creation of the diff.
+	// The media type which is used to determine the format of the created
+	// content can also be provided as an option.
+	DiffMounts(ctx context.Context, lower, upper []mount.Mount, opts ...Opt) (ocispec.Descriptor, error)
+}
+
+// WithMediaType sets the media type to use for creating the diff, without
+// specifying the differ will choose a default.
+func WithMediaType(m string) Opt {
+	return func(c *Config) error {
+		c.MediaType = m
+		return nil
+	}
+}
+
+// WithReference is used to set the content upload reference used by
+// the diff operation. This allows the caller to track the upload through
+// the content store.
+func WithReference(ref string) Opt {
+	return func(c *Config) error {
+		c.Reference = ref
+		return nil
+	}
+}
+
+// WithLabels is used to set content labels on the created diff content.
+func WithLabels(labels map[string]string) Opt {
+	return func(c *Config) error {
+		c.Labels = labels
+		return nil
+	}
+}