Fix syncTrees from generating too many patches

This is a reversion of the change found in
e282dbc and a test to ensure
the prevention of future regressions.

packages/diffhtml/lib/tree/sync.js

@@ -71,7 +71,7 @@ export default function syncTree(oldTree, newTree, patches, parentTree, specialC
     // then splice it into the parent (if it exists) and run a sync.
     if (retVal && retVal !== newTree) {
       newTree.childNodes = [].concat(retVal);
-      syncTree(null, retVal, patches, newTree);
+      syncTree(oldTree !== empty ? oldTree : null, retVal, patches, newTree);
       newTree = retVal;
     }
   });

packages/diffhtml/test/tree.js

@@ -8,6 +8,8 @@ import {
 } from 'assert';
 import createTree from '../lib/tree/create';
 import syncTree from '../lib/tree/sync';
+import { MiddlewareCache } from '../lib/util/caches';
+import parse from '../lib/util/parse';
 import validateMemory from './util/validateMemory';
 
 describe('Tree', function() {
@@ -714,6 +716,32 @@ describe('Tree', function() {
         });
       });
 
+      it('will not generate patches when returning old element', () => {
+        const hook = (oldTree, newTree) =>
+          oldTree && oldTree.attributes && oldTree.attributes.class === 'text' ?
+            oldTree :
+            newTree;
+        MiddlewareCache.SyncTreeHookCache.add(hook);
+
+        const oldTree = parse(`
+          <div class="parent"><div class="child"><span class="text">Hello world!</span></div></div>
+        `).childNodes[0];
+        const newTree = parse(`
+          <div class="parent"><div class="child"><span class="image">Goodbye world!</span></div></div>
+        `).childNodes[0];
+
+        const patches = syncTree(oldTree, newTree);
+
+        deepEqual(patches, {
+          TREE_OPS: [],
+          NODE_VALUE: [],
+          SET_ATTRIBUTE: [],
+          REMOVE_ATTRIBUTE: [],
+        });
+
+        MiddlewareCache.SyncTreeHookCache.delete(hook);
+      });
+
       describe.skip('Attribute & Property Differences', () => {
         it('will convert className to class', () => {
           const oldTree = createTree('div');