[FIX] fields: combine special values during onchange

Consider a many2one field `foo_id` on model `bar`, with an inverse one2many
field `bar_ids` on model `foo`.  During an onchange, the statement

    bar.foo_id = foo

puts a special value in cache to add `bar` to the value of `foo.bar_ids`
without explicitly reading `foo.bar_ids`.

Executing the above statement a second time, the cache of `foo.bar_ids` is no
longer empty.  This causes the actual value of `foo.bar_ids` to be read and
updated.  The issue is that this can be slow for large values of `foo.bar_ids`.

Avoid reading the value of the one2many field by handling the case where the
cache contains the special value: simply update the special value to take into
account the second assignment.

closes odoo/odoo#28982

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