ui: optimize buffered write with no label

This was spotted while making fastannotate faster again after ditching its
own formatter. Since I'm going to inline _write() into ui.write(), I decided
to include this patch in this series.

Here, the cost of '(self.label(a, label) for a in args)' was significant
in hot loops.

mercurial/ui.py

@@ -1006,8 +1006,8 b' class ui(object):'
 
     def _write(self, dest, *args, **opts):
         if self._isbuffered(dest):
-            if self._bufferapplylabels:
                 label = opts.get(r'label', '')
+            if label and self._bufferapplylabels:
                 self._buffers[-1].extend(self.label(a, label) for a in args)
             else:
                 self._buffers[-1].extend(args)