[FIX] fields: check if the inverse must change

Before this commit, when modifying a Many2one in a form view to add a
new record,  it will create the new record, and calculate the inverse
and write it in all the records of the list. This will call the write
method in all records even if they weren't changed.

Now, the inverse is set to be modified only if it's different from the
current value.

opw-2091842

closes odoo/odoo#39927

Signed-off-by: Jorge Pinna Puissant (jpp) <jpp@odoo.com>
Co-authored-by: Raphaël Collet <rco@odoo.com>