Update utility "bin/repository parents" workflow to work with RefPosition

Summary:
Ref T11823. I think this is the last callsite which relies on the old data format: `bin/repository parents` rebuilds a cache which we don't currently use very heavily.

Update it to work with the new data.

Test Plan: Ran `bin/repository parents <repository> --trace`, saw successful script execution and reasonable-looking output.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T11823

Differential Revision: https://secure.phabricator.com/D18615

src/applications/repository/management/PhabricatorRepositoryManagementParentsWorkflow.php

@@ -54,6 +54,7 @@ final class PhabricatorRepositoryManagementParentsWorkflow
       ->setViewer($this->getViewer())
       ->withRefTypes(array(PhabricatorRepositoryRefCursor::TYPE_BRANCH))
       ->withRepositoryPHIDs(array($repo->getPHID()))
+      ->needPositions(true)
       ->execute();
 
     $graph = array();
@@ -66,8 +67,7 @@ final class PhabricatorRepositoryManagementParentsWorkflow
         "%s\n",
         pht('Rebuilding branch "%s"...', $ref->getRefName()));
 
-      $commit = $ref->getCommitIdentifier();
-
+      foreach ($ref->getPositionIdentifiers() as $commit) {
         if ($repo->isGit()) {
           $stream = new PhabricatorGitGraphStream($repo, $commit);
         } else {
@@ -86,6 +86,7 @@ final class PhabricatorRepositoryManagementParentsWorkflow
           }
         }
       }
+    }
 
     $console->writeOut(
       "%s\n",