Update widgets for attributes which have a data provider default value

- Apply for widgets which show a key/description instead of the actual
  value

src/app/qgsmergeattributesdialog.cpp

@@ -267,6 +267,15 @@ void QgsMergeAttributesDialog::createTableWidgetContents()
         currentComboBox->setCurrentIndex( currentComboBox->findData( QStringLiteral( "manual" ) ) );
         currentComboBox->blockSignals( false );
       }
+
+      const QString widgetType = QgsGui::editorWidgetRegistry()->findBest( mVectorLayer, mFields.at( idx ).name() ).type();
+
+      const QStringList updateWidgetFor = QStringList() << QStringLiteral( "ValueMap" )
+                                                        << QStringLiteral( "ValueRelation" )
+                                                        << QStringLiteral( "RelationReference" );
+
+      if ( updateWidgetFor.contains( widgetType ) )
+        updateManualWidget( j, true );
     }
   }