-//
-// TODO(TC): Before returning CollisionError, compute the MD5 digest
-// of the data on disk (i.e., known-to-be-equal data in cmp +
-// remaining data on disk) and return DiskHashError instead of
-// CollisionError if it doesn't equal loc[:32].
-func (v *UnixVolume) Compare(loc string, cmp []byte) error {