[16.0][FIX] mail_improved_tracking_value: support tracking Many2one field

[trisdoan]

Steps to reproduce

1. Change value Many2one field
2. Open tracking value form: Setting -> Technical -> Email -> Improved Tracking Values
3. Get this error

ValueError: Compute method failed to assign mail.tracking.value(1,).old_value_formatted

[StefanRijnhart]

There are tests for all the other field types. Can you add a similar test for Many2one?

[github-actions[bot]]

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