Use ExportChanges() in runtime.Diff()

This code was duplicated in two places, one which was unused. This
syncs the code and removes the unused version.

archive/changes.go

@@ -280,11 +280,7 @@ func ChangesDirs(newDir, oldDir string) ([]Change, error) {
 	return newRoot.Changes(oldRoot), nil
 }
 
-func ExportChanges(root, rw string) (Archive, error) {
-	changes, err := ChangesDirs(root, rw)
-	if err != nil {
-		return nil, err
-	}
+func ExportChanges(dir string, changes []Change) (Archive, error) {
 	files := make([]string, 0)
 	deletions := make([]string, 0)
 	for _, change := range changes {
@@ -297,5 +293,11 @@ func ExportChanges(root, rw string) (Archive, error) {
 			deletions = append(deletions, filepath.Join(dir, ".wh."+base))
 		}
 	}
-	return TarFilter(root, &TarOptions{Compression: Uncompressed, Recursive: false, Includes: files, CreateFiles: deletions})
+	// FIXME: Why do we create whiteout files inside Tar code ?
+	return TarFilter(dir, &TarOptions{
+		Compression: Uncompressed,
+		Includes:    files,
+		Recursive:   false,
+		CreateFiles: deletions,
+	})
 }

runtime.go

@@ -18,7 +18,6 @@ import (
 	"os"
 	"os/exec"
 	"path"
-	"path/filepath"
 	"sort"
 	"strings"
 	"time"
@@ -763,25 +762,7 @@ func (runtime *Runtime) Diff(container *Container) (archive.Archive, error) {
 		return nil, fmt.Errorf("Error getting container rootfs %s from driver %s: %s", container.ID, container.runtime.driver, err)
 	}
 
-	files := make([]string, 0)
-	deletions := make([]string, 0)
-	for _, change := range changes {
-		if change.Kind == archive.ChangeModify || change.Kind == archive.ChangeAdd {
-			files = append(files, change.Path)
-		}
-		if change.Kind == archive.ChangeDelete {
-			base := filepath.Base(change.Path)
-			dir := filepath.Dir(change.Path)
-			deletions = append(deletions, filepath.Join(dir, ".wh."+base))
-		}
-	}
-	// FIXME: Why do we create whiteout files inside Tar code ?
-	return archive.TarFilter(cDir, &archive.TarOptions{
-		Compression: archive.Uncompressed,
-		Includes:    files,
-		Recursive:   false,
-		CreateFiles: deletions,
-	})
+	return archive.ExportChanges(cDir, changes)
 }
 
 func linkLxcStart(root string) error {