Make sure all 301 url redirects get generated when importing more then one product at the same time while using the 'save_rewrites_history' flag.

Author: hostep

app/code/Magento/CatalogUrlRewrite/Observer/AfterImportDataObserver.php

@@ -475,17 +475,18 @@ private function currentUrlRewritesRegenerate()
             ]
         );
 
-        $urlRewrites = [];
+        $mergeDataProvider = clone $this->mergeDataProviderPrototype;
         foreach ($currentUrlRewrites as $currentUrlRewrite) {
             $category = $this->retrieveCategoryFromMetadata($currentUrlRewrite);
             if ($category === false) {
                 continue;
             }
-            $url = $currentUrlRewrite->getIsAutogenerated()
+            $urls = $currentUrlRewrite->getIsAutogenerated()
                 ? $this->generateForAutogenerated($currentUrlRewrite, $category)
                 : $this->generateForCustom($currentUrlRewrite, $category);
-            $urlRewrites = $url + $urlRewrites;
+            $mergeDataProvider->merge($urls);
         }
+        $urlRewrites = $mergeDataProvider->getData();
 
         $this->product = null;
         $this->productCategories = null;