FIX: automatically timeout long running image magick commands (#12670)

Previously certain images may lead to convert / identify to run for unreasonable
amounts of time

This adds a maximum amount of time these commands can run prior to forcing
them to stop

lib/theme_store/git_importer.rb

@@ -82,7 +82,7 @@ class ThemeStore::GitImporter
       else
         Discourse::Utils.execute_command("git", "clone", @url, @temp_folder)
       end
-    rescue RuntimeError => err
+    rescue RuntimeError
       raise RemoteTheme::ImportError.new(I18n.t("themes.import_error.git"))
     end
   end