[FIX] crm: handle selection fields correctly while merging leads

When merging two or more leads, a traceback was generated when attempting
to merge some selection field, if any.

closes https://github.com/odoo/odoo/issues/25644

opw 1864870

addons/crm/models/crm_lead.py

@@ -504,7 +504,8 @@ class Lead(FormatAddress, models.Model):
         for field in fields:
             value = getattr(self, field.name, False)
             if field.ttype == 'selection':
-                value = dict(field.get_values(self.env)).get(value, value)
+                selections = self.fields_get()[field.name]['selection']
+                value = next((v[1] for v in selections if v[0] == value), value)
             elif field.ttype == 'many2one':
                 if value:
                     value = value.sudo().name_get()[0][1]