Dashboard Tabs: fix action "Move Right"

Summary:
It seems that if you use the "Move Right" action, you can get
an unrelated error message about "Move Left":

    This is already the first tab. It can not move any farther to the left.

This happened because there was an incomplete condition.

Closes T15493

Test Plan:
- visit /dashboard/panel/ and create at least two panels (please, owned by you)
- visit a Dashboard, and add a Tabs Panel
- take the first Tab: "Move Right"

After this change, "Move Right" now always works.

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: speck, tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15493

Differential Revision: https://we.phorge.it/D25307

src/applications/dashboard/controller/panel/PhabricatorDashboardPanelTabsController.php

@@ -327,7 +327,7 @@ final class PhabricatorDashboardPanelTabsController
             'This is already the last tab. It can not move any farther to '.
             'the right.'))
         ->addCancelButton($cancel_uri);
-    } else if ((string)head_key($old_config) === $target) {
+    } else if (!$is_next && (string)head_key($old_config) === $target) {
       return $this->newDialog()
         ->setTitle(pht('Impossible!'))
         ->appendParagraph(