I-3773: Fixed drush_backend_batch_process() not clearing batch_get(). (…

…drush-ops#4886)

Co-authored-by: Francesco Placella <plach@183211.no-reply.drupal.org>

includes/batch.inc

@@ -159,6 +159,11 @@ function _drush_backend_batch_process($command = 'batch-process', $args = [], $o
     }
   }
 
+  // An attempt to clear the batch is performed in "_drush_batch_finished",
+  // however when that is run in a separate process, static cache changes need
+  // to be replayed.
+  $batch = NULL;
+
   return $result;
 }