[FIX] contract: do not die if modified partner has contracts in several companies

In a multicompany scenario where a contact belongs to a company and has contracts in several companies, if the user goes to the contact to edit anything, when saving, it will trigger the recomputation of the `commercial_partner_id` field, which gets populated to all the related contracts, and will undeniably fail with an `AccessError`.

A simple test is provided that, without the fix, fails like this:

```
2020-06-25 07:21:24,859 33 ERROR devel odoo.addons.contract.tests.test_contract: ERROR: test_multicompany_partner_edited (odoo.addons.contract.tests.test_contract.TestContract)
2020-06-25 07:21:24,859 33 ERROR devel odoo.addons.contract.tests.test_contract: ` Editing a partner with contracts in several companies works.
2020-06-25 07:21:24,859 33 ERROR devel odoo.addons.contract.tests.test_contract: Traceback (most recent call last):
2020-06-25 07:21:24,859 33 ERROR devel odoo.addons.contract.tests.test_contract: `   File "/opt/odoo/auto/addons/contract/tests/test_contract.py", line 2513, in test_multicompany_partner_edited
2020-06-25 07:21:24,859 33 ERROR devel odoo.addons.contract.tests.test_contract: `     'parent_id': parent_partner.id,
2020-06-25 07:21:24,859 33 ERROR devel odoo.addons.contract.tests.test_contract: `   File "/opt/odoo/auto/addons/partner_autocomplete/models/res_partner.py", line 183, in write
2020-06-25 07:21:24,860 33 ERROR devel odoo.addons.contract.tests.test_contract: `     res = super(ResPartner, self).write(values)
2020-06-25 07:21:24,860 33 ERROR devel odoo.addons.contract.tests.test_contract: `   File "/opt/odoo/custom/src/odoo/odoo/addons/base/models/res_partner.py", line 570, in write
2020-06-25 07:21:24,860 33 ERROR devel odoo.addons.contract.tests.test_contract: `     result = result and super(Partner, self).write(vals)
2020-06-25 07:21:24,860 33 ERROR devel odoo.addons.contract.tests.test_contract: `   File "/opt/odoo/auto/addons/mail/models/mail_thread.py", line 321, in write
2020-06-25 07:21:24,860 33 ERROR devel odoo.addons.contract.tests.test_contract: `     result = super(MailThread, self).write(values)
2020-06-25 07:21:24,860 33 ERROR devel odoo.addons.contract.tests.test_contract: `   File "/opt/odoo/auto/addons/mail/models/mail_activity.py", line 613, in write
2020-06-25 07:21:24,860 33 ERROR devel odoo.addons.contract.tests.test_contract: `     return super(MailActivityMixin, self).write(vals)
2020-06-25 07:21:24,860 33 ERROR devel odoo.addons.contract.tests.test_contract: `   File "/opt/odoo/custom/src/odoo/odoo/models.py", line 3381, in write
2020-06-25 07:21:24,860 33 ERROR devel odoo.addons.contract.tests.test_contract: `     self.recompute()
2020-06-25 07:21:24,860 33 ERROR devel odoo.addons.contract.tests.test_contract: `   File "/opt/odoo/custom/src/odoo/odoo/models.py", line 5308, in recompute
2020-06-25 07:21:24,860 33 ERROR devel odoo.addons.contract.tests.test_contract: `     target._write(dict(vals))
2020-06-25 07:21:24,860 33 ERROR devel odoo.addons.contract.tests.test_contract: `   File "/opt/odoo/custom/src/odoo/odoo/models.py", line 3433, in _write
2020-06-25 07:21:24,860 33 ERROR devel odoo.addons.contract.tests.test_contract: `     self.check_access_rule('write')
2020-06-25 07:21:24,860 33 ERROR devel odoo.addons.contract.tests.test_contract: `   File "/opt/odoo/custom/src/odoo/odoo/models.py", line 3081, in check_access_rule
2020-06-25 07:21:24,860 33 ERROR devel odoo.addons.contract.tests.test_contract: `     + ' - ({} {}, {} {})'.format(_('Records:'), invalid.ids[:6], _('User:'), self._uid)
2020-06-25 07:21:24,861 33 ERROR devel odoo.addons.contract.tests.test_contract: ` odoo.exceptions.AccessError: ('The requested operation cannot be completed due to security restrictions. Please contact your system administrator.\n\n(Document type: Contract, Operation: write) - (Records: [101], User: 12)', None)
```

@Tecnativa TT24482

contract/models/contract.py

@@ -87,6 +87,7 @@ class ContractContract(models.Model):
 
     commercial_partner_id = fields.Many2one(
         'res.partner',
+        compute_sudo=True,
         related='partner_id.commercial_partner_id',
         store=True,
         string='Commercial Entity',