X2many fields tracking was not supported in the implementation of
`mail` module:
* not needed in the past
* the logic was not hooked in the same place before. Now that it is
managed pre-commit, x2many values don't have to be handled as commands,
but can be compared as records.
* ...
The support was added for two specific models in specific modules, but
it's a good opportunity to clean that and to support it directly in `mail`.
Task-3316528
Part-of: odoo/odoo#121244
Valeriya(vchu)
167944cc93