[FIX] fields: performance of convert_to_onchange on x2many fields
[FIX] fields: performance of convert_to_onchange on x2many fields

Suppose we convert a x2many field `foo_ids` with a many2one sub-field `bar_id`.
The conversion of the values of `bar_id` generates pairs `(id, name)`.  Before
this patch, the method `name_get` is invoked on every value, one by one.
Rewrite the code to ensure that `name_get()` is invoked on all values at once.

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