When using "bin/bulk export --overwrite", actually overwrite the file

Summary: Depends on D19738. Ref T13210. Currently, when you use "--overwrite", we just //append// the new content. Instead, actually overwrite the file.

Test Plan: Used `--overwrite`, saw an actual clean overwrite instead of an append.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13210

Differential Revision: https://secure.phabricator.com/D19739

Author: epriestley

src/applications/transactions/bulk/management/PhabricatorBulkManagementExportWorkflow.php

@@ -114,6 +114,10 @@ public function execute(PhutilArgumentParser $args) {
     $iterator = $file->getFileDataIterator();
 
     if (strlen($output_path)) {
+      // Empty the file before we start writing to it. Otherwise, "--overwrite"
+      // will really mean "--append".
+      Filesystem::writeFile($output_path, '');
+
       foreach ($iterator as $chunk) {
         Filesystem::appendFile($output_path, $chunk);
       }