layerStore.Diff(): don't use an unnecessary buffer

Don't ReadAll() from a Reader to create a buffer and then create another
Reader to read from that buffer.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

layers.go

@@ -1471,12 +1471,7 @@ func (r *layerStore) Diff(from, to string, options *DiffOptions) (io.ReadCloser,
 	}
 	defer decompressor.Close()
 
-	tsbytes, err := ioutil.ReadAll(decompressor)
-	if err != nil {
-		return nil, err
-	}
-
-	metadata = storage.NewJSONUnpacker(bytes.NewBuffer(tsbytes))
+	metadata = storage.NewJSONUnpacker(decompressor)
 
 	fgetter, err := r.newFileGetter(to)
 	if err != nil {