Fix breaking because of D935

Summary:
D935 missed one place of parseGitRelativeCommit() in
ArcanistExportWorkflow.

Test Plan: ran arc export and verify that it worked.

Reviewers: epriestley, tuomaspelkonen, aran

Reviewed By: aran

CC: aran

Differential Revision: 1015

src/workflow/export/ArcanistExportWorkflow.php

@@ -173,8 +173,7 @@ EOTEXT
         $parser = new ArcanistDiffParser();
 
         if ($repository_api instanceof ArcanistGitAPI) {
-          $this->parseGitRelativeCommit(
-            $repository_api,
+          $repository_api->parseRelativeLocalCommit(
             $this->getArgument('paths'));
           $diff = $repository_api->getFullGitDiff();
           $changes = $parser->parseDiff($diff);