extensionDownloader: Exclude extensions with pending updates from check

While it is possible that an extension has a newer version available
than the previously downloaded update, it's more likely that we end up
downloading the same archive again. That would be a bit silly despite
the usually small size, so we can either use the metadata from the
update, or exclude the extension from the check.

The latter is much easier, so let's go with that for now.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/945

js/ui/extensionDownloader.js

@@ -123,6 +123,8 @@ function checkForUpdates() {
         let extension = Main.extensionManager.lookup(uuid);
         if (extension.type !== ExtensionUtils.ExtensionType.PER_USER)
             return;
+        if (extension.hasUpdate)
+            return;
         metadatas[uuid] = extension.metadata;
     });