[FIX] crm: Access rights issues in contact merging

- When merging partners with fields restricted to certain groups which the user doesn't belong, an access error is raised.
  To avoid this, we only merge fields that are accessible by the user. We do so by using the method fields_get() that only returns the fields accessible by the current user.

addons/crm/wizard/base_partner_merge.py

@@ -223,7 +223,7 @@ class MergePartnerAutomatic(models.TransientModel):
         """
         _logger.debug('_update_values for dst_partner: %s for src_partners: %r', dst_partner.id, src_partners.ids)
 
-        model_fields = dst_partner._fields
+        model_fields = dst_partner.fields_get().keys()
 
         def write_serializer(item):
             if isinstance(item, models.BaseModel):
@@ -232,7 +232,8 @@ class MergePartnerAutomatic(models.TransientModel):
                 return item
         # get all fields that are not computed or x2many
         values = dict()
-        for column, field in model_fields.items():
+        for column in model_fields:
+            field = dst_partner._fields[column]
             if field.type not in ('many2many', 'one2many') and field.compute is None:
                 for item in itertools.chain(src_partners, [dst_partner]):
                     if item[column]: