[FIX] core: convert_to_write() of x2many field with new records

When comparing the values of a new record with its origin, x2many fields
are always different because we compare new records with real records.
For instance, converting the value of a one2many field where lines have
a many2many field always returns update commands for the many2many
field.

closes odoo/odoo#66914

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