account for symlinks

[yarikoptic]

another "complication" -- for datalad we would need to also account for either symlink (if symlink) is broken or not to provide relevant status. For git it doesn't matter. So I guess should be an option to specific instance, and if symlinks='add-followed' (or alike), then their record/fingerprint should also contain also stat for a de-referenced (all the way?) file