[FIX] mail: consider more than last 1000 activities

Because activities_to_check was assigned in each loop, the result was
only containing the last chunk of 1000 records, instead of the whole
result.

This commit fixes the issue by concatenating into the list instead of
just assigning it.

X-original-commit: c06c96655b88cc4df59b8cdd5f05a2f600255c49
Part-of: odoo/odoo#79969

addons/mail/models/mail_activity.py

@@ -357,7 +357,7 @@ class MailActivity(models.Model):
                 SELECT DISTINCT activity.id, activity.res_model, activity.res_id
                 FROM "%s" activity
                 WHERE activity.id = ANY (%%(ids)s)""" % self._table, dict(ids=list(sub_ids)))
-            activities_to_check = self._cr.dictfetchall()
+            activities_to_check += self._cr.dictfetchall()
 
         activity_to_documents = {}
         for activity in activities_to_check: