Fix duplicating panel when editing in column 2

Summary: Fixes T10612. We're writing a new panel to any dashboard even if it already exists. No need when just updating a panel title.

Test Plan: Add "welcome" panel to column 2 of a clean dashboard. Edit title, save. See correct panel in correct place.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T10612

Differential Revision: https://secure.phabricator.com/D17349

src/applications/dashboard/controller/PhabricatorDashboardPanelEditController.php

@@ -143,7 +143,7 @@ final class PhabricatorDashboardPanelEditController
           ->applyTransactions($panel, $xactions);
 
         // If we're creating a panel directly on a dashboard, add it now.
-        if ($dashboard) {
+        if ($dashboard && $is_create) {
           PhabricatorDashboardTransactionEditor::addPanelToDashboard(
             $viewer,
             PhabricatorContentSource::newFromRequest($request),