Stop calling the undefined withIsTag method

Summary: This just cleans up a method call that was missed in D15986. It's been causing fatal errors in one of our workflows.

Test Plan: Grep'd for other instances of `withIsTag` and didn't find any

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, yelirekim

Differential Revision: https://secure.phabricator.com/D17299

src/applications/diffusion/query/lowlevel/DiffusionLowLevelResolveRefsQuery.php

@@ -178,7 +178,10 @@ final class DiffusionLowLevelResolveRefsQuery
       // repositories with a huge number of tags.
       $tag_refs = id(new DiffusionLowLevelGitRefQuery())
         ->setRepository($repository)
-        ->withIsTag(true)
+        ->withRefTypes(
+          array(
+            PhabricatorRepositoryRefCursor::TYPE_TAG,
+          ))
         ->executeQuery();
       foreach ($tag_refs as $tag_ref) {
         $tag_map[$tag_ref->getShortName()] = $tag_ref->getCommitIdentifier();