[FIX] base: keep activity after partner merged

Steps to reproduce:
- install contacts
- go to any 2 contacts and add a message and an activity to the chatter
- go to the list view > select the 2 contacts > action > merge
- click on the remaining contact and refresh

Previous behavior:
the activities have not been merged

Current behavior:
activities are merged

opw-2263853

closes odoo/odoo#52597

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>

odoo/addons/base/wizard/base_partner_merge.py

@@ -196,6 +196,7 @@ class MergePartnerAutomatic(models.TransientModel):
             update_records('calendar', src=partner, field_model='model_id.model')
             update_records('ir.attachment', src=partner, field_model='res_model')
             update_records('mail.followers', src=partner, field_model='res_model')
+            update_records('mail.activity', src=partner, field_model='res_model')
             update_records('mail.message', src=partner)
             update_records('ir.model.data', src=partner)