Snapshot and Timestamp default content

Snapshot and Timestamp constructors try to be clever:

self.meta = meta if meta is not None else {"targets.json": MetaFile(1)}

and

self.snapshot_meta = snapshot_meta or MetaFile(1)

So they set the metafile content without knowing what it really should be

this is annoying as Repository.snapshot() and Repository.timestamp() now think a snapshot and timestamp are not needed -- even though none have been generated yet.

I can work around this but especially for snapshot the default value seems wrong: empty dict would be more correct -- the meta dict should be filled by a conscious decision not by a default value that might be right.

For timestamp there might not be a correct value though. I think MetaFile(0) would be best (although it requires loosening the check for valid MetaFiles): it's at least clear that timestamp doesn't have a snapshot version yet

Inizia leggendo i costruttori di Snapshot e Timestamp e i metodi Repository.snapshot() e Repository.timestamp() per capire in che modo i relativi metadati predefiniti influenzano i controlli di generazione. Confronta i valori predefiniti proposti empty-dict e MetaFile(0) con le regole di validità esistenti di MetaFile; il lavoro è completato quando gli oggetti appena costruiti non indicano erroneamente che siano stati generati metadati di snapshot o timestamp.