Fix a Calendar import issue where we looked up attendees by object instead of name

Summary:
Ref T11801. This issue led to the stack trace in T11801#199042.

It wasn't obvious that this was wrong because the recover-on-duplicate-key code made it work correctly.

Test Plan: Imported an event with external attendees with no warnings in the log.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11801

Differential Revision: https://secure.phabricator.com/D16804

src/applications/calendar/import/PhabricatorCalendarImportEngine.php

@@ -259,7 +259,7 @@ abstract class PhabricatorCalendarImportEngine
     if ($attendee_names) {
       $external_invitees = id(new PhabricatorCalendarExternalInviteeQuery())
         ->setViewer($viewer)
-        ->withNames($attendee_names)
+        ->withNames(array_keys($attendee_names))
         ->execute();
       $external_invitees = mpull($external_invitees, null, 'getName');