[FIX] core: avoid multiple calls to default_get() in onchange

Upon the first call to `onchange`, the `values` dict is filled with
default values.  But the fields without a default are left unassigned.
This allows to compute fields when accessing them, but non-computed
fields will ask again for their default value, which is a waste of time
because we know they don't have one.  This may even be a performance
issue on forms with many fields.

Fix this issue by completing `values` with falsy values, and consider
the fields with some initial value (given or default) as the assigned
ones for triggering onchanges.  This explicitly triggers the computation
of fields that depend on fields with some initial value, without forcing
them to `False`.

Also avoid accessing x2many fields on the record in `snapshot.diff()`.
Enable the prefetching by simply putting the right value in cache.

closes odoo/odoo#62627

Signed-off-by: Raphael Collet (rco) <rco@openerp.com>

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