specify serialization format for repository snapshot objects

We need to decide between a-la Software Heritage and a-la Git serialization formats (see blueprint).

• argument for a-la Software Heritage is that we can make it cleaner and more sane minded than Git idiosyncrasies, e.g., when it comes to separators, entry order, etc
• argument for a-la Git is that it will be coherence with the fact that other objects are serialized following Git conventions

Once we have decided this, the spec (and the blueprint) should be (more) permanently stored in a Git repo, most likely rDMOD.

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Migrated from T566 (view on Phabricator)