purchasing: avoid multiple items creation

phabricator creates xactions as previews when someone is editing
the tickect through the comments section. We do not want to trigger
the asset creation in Snipe IT in theses cases, otherwise we will
end up with duplicated items in snipe IT.

src/applications/maniphest/editor/ManiphestTransactionEditor.php

@@ -395,8 +395,8 @@ final class ManiphestTransactionEditor
           $new_status = $xaction->getNewValue();
 
           // CCU: create Item in the inventory system if it's a Purchase request and
-          // its status was changed to "resolved" (Closed)
-          if (!$this->getIsNewObject()) {
+          // its status was changed to "resolved" (Closed) and it's not a preview
+          if (!$this->getIsNewObject() && !$this->getIsPreview()) {
             try {
               $asset_transactions = $snipe_api->createAssetItem($new_status, $object, $xactions);
             } catch (Exception $e) {