Replace the existing file

Created by: liyakun

When you need to edit some file of your project, then you need to edit it. One function to replace the existing file is added, which enable user to choose a new file to replace the existing one, and the name of the new file will be the same as previous one. The UI is changed also, please check the attachment.

Existing file "new_tree_controller.rb", "_actions.html.haml" and "show.html.haml" are changed. New file "_update.html.haml" is created.