diff: rename differ to comparer

Remove combined interface and split implementations.

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

diff/diff.go

@@ -1,9 +1,10 @@
 package diff
 
 import (
+	"context"
+
 	"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
@@ -24,14 +25,14 @@ type Config struct {
 // Opt is used to configure a diff operation
 type Opt func(*Config) error
 
-// Differ allows creation of filesystem diffs between mounts
-type Differ interface {
-	// Diff computes the difference between two mounts and returns a
+// Comparer allows creation of filesystem diffs between mounts
+type Comparer interface {
+	// Compare 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.
-	Diff(ctx context.Context, lower, upper []mount.Mount, opts ...Opt) (ocispec.Descriptor, error)
+	Compare(ctx context.Context, lower, upper []mount.Mount, opts ...Opt) (ocispec.Descriptor, error)
 }
 
 // Applier allows applying diffs between mounts
@@ -44,15 +45,6 @@ type Applier interface {
 	Apply(ctx context.Context, desc ocispec.Descriptor, mount []mount.Mount) (ocispec.Descriptor, error)
 }
 
-// DiffApplier is the interface that groups the basic Apply and Diff methods.
-//
-// golint says `type name will be used as diff.DiffApplier by other packages, and that stutters`,
-// but that can be ignored now.
-type DiffApplier interface { // nolint: golint
-	Applier
-	Differ
-}
-
 // WithMediaType sets the media type to use for creating the diff, without
 // specifying the differ will choose a default.
 func WithMediaType(m string) Opt {