func (fs *MemFS) merge(l *memLayer) error {
	fs.layers = append(fs.layers, l)
	if err := l.rangeFiles(func(f memFile) error {
		return f.updateMemFS(fs.tree)
	}); err != nil {
		return fmt.Errorf("range files: %s", err)
	}
	return nil
}
merge merges the given layer into MemFS. This is only used for testing, as
files are merged as the layer is created when adding from a scan/copy.
package snapshot
import (
	"archive/tar"
	"fmt"
	"io"
	"os"
	"path/filepath"
	"strings"
	"time"
	"github.com/uber/makisu/lib/tario"
	"github.com/stretchr/testify/require"
)