cmdutil: pass labeled=True to pushbuffer()

This doesn't yet change behavior because labeling is still performed
at popbuffer time.

Surprisingly, this is the only in-tree consumer that passes
labeled=True.

mercurial/cmdutil.py

@@ -1186,7 +1186,7 b' class changeset_printer(object):'
 
     def show(self, ctx, copies=None, matchfn=None, **props):
         if self.buffered:
-            self.ui.pushbuffer()
+            self.ui.pushbuffer(labeled=True)
             self._show(ctx, copies, matchfn, props)
             self.hunk[ctx.rev()] = self.ui.popbuffer(labeled=True)
         else: