bundle: when forcing acceptance of incoming delta also accept snapshot
Snapshot where never considered reusable and the unbundling side always tried
to find a delta from them. In the `forced` mode this is counter-productive
because it will either connect two delta-tree that should not be connected or
it will spend potentially a lot of time because creating a full snapshot
anyway.
So in this mode, we accept the full snapshot as is.
This changeset is benchmarked with its children so please do not split them
apart when landing.
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