Speed up listing files in add mode

I have a pretty big project (more then 30000 files) where creating the list after pressing CTRL-A is very slow. Takes a minute or more to get the list.

I figured out that the slowest part is to create the string which represents the files.

I moved out these methods into ruby, which did a big impact on performance.
The patch is looking pretty ugly, because there are so many arguments need to be passed to the methods, so probably it needs some more work.
I would like the changes merged or if you have suggestions how can we make it better I would do it to!

Thanks for the plugin!