Make WPF XAML compiler not crash.

Seems you can't assign text directly to the `Content` of a subclass of
`UserControl`.

Author: grokys

src/GitHub.VisualStudio/UI/Views/NotAGitHubRepositoryView.xaml

@@ -21,5 +21,5 @@
     </ResourceDictionary>
   </UserControl.Resources>
 
-    Not a GitHub repository.
+  <TextBlock HorizontalAlignment="Center">Not a GitHub repository.</TextBlock>
 </local:GenericNotAGitHubRepositoryView>