Readme rendering fix

Created by: dfyx

Quick fix for #1676 (closed). A more elegant solution would be to share code between app/views/tree/_readme.html.haml and app/views/blob/_text.html.haml but I'm not that good with HAML and it works for now.

app/helpers/tree_helper.rb

@@ -54,7 +54,7 @@ module TreeHelper
   end
   def plain_text_readme? filename
-    filename == 'README'
+    filename =~ /^README(\.txt)?$/i
   end
   # Simple shortcut to File.join