Use update() instead of repaint() when displaying a new message

There is no advantage to using repaint() here, so using update will
give a performance improvement.

Change-Id: Icc6a28dfc12dffb8ea3df0300fd14c66c775bf16
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

src/widgets/widgets/qstatusbar.cpp

@@ -643,7 +643,7 @@ void QStatusBar::hideOrShow()
     }
 #endif
 
-    repaint(d->messageRect());
+    update(d->messageRect());
 }
 
 /*!