Optimise selection.order.

A small optimisation: only call insertBefore() if node.nextSibling isn't
already what we want, so that document nodes that are already in the
correct order can be skipped.

d3.js

@@ -1777,7 +1777,7 @@ d3_selectionPrototype.order = function() {
   for (var j = -1, m = this.length; ++j < m;) {
     for (var group = this[j], i = group.length - 1, next = group[i], node; --i >= 0;) {
       if (node = group[i]) {
-        if (next) next.parentNode.insertBefore(node, next);
+        if (next && next !== node.nextSibling) next.parentNode.insertBefore(node, next);
         next = node;
       }
     }

src/core/selection-order.js

@@ -2,7 +2,7 @@ d3_selectionPrototype.order = function() {
   for (var j = -1, m = this.length; ++j < m;) {
     for (var group = this[j], i = group.length - 1, next = group[i], node; --i >= 0;) {
       if (node = group[i]) {
-        if (next) next.parentNode.insertBefore(node, next);
+        if (next && next !== node.nextSibling) next.parentNode.insertBefore(node, next);
         next = node;
       }
     }