Don't remove trailing newline when editing a file in web UI editor:

* populate ACE editor content via API instead of HTML element inner text (to preserve trailing whitespace).
* remove hack in EditTreeController#preview that replaces any removed \n.

Fixes #5936

app/controllers/projects/edit_tree_controller.rb

@@ -28,8 +28,6 @@ class Projects::EditTreeController < Projects::BaseTreeController
 
   def preview
     @content = params[:content]
-    #FIXME workaround https://github.com/gitlabhq/gitlabhq/issues/5936
-    @content += "\n" if @blob.data.end_with?("\n")
 
     diffy = Diffy::Diff.new(@blob.data, @content, diff: '-U 3',
                             include_diff_info: true)

app/views/projects/edit_tree/show.html.haml

@@ -16,7 +16,7 @@
           .btn-group.tree-btn-group
             = link_to "Cancel", @after_edit_path, class: "btn btn-tiny btn-cancel", data: { confirm: leave_edit_message }
       .file-content.code
-        %pre.js-edit-mode-pane#editor= @blob.data
+        %pre.js-edit-mode-pane#editor
         .js-edit-mode-pane#preview.hide
           .center
             %h2
@@ -43,6 +43,7 @@
   ace.config.set("modePath", gon.relative_url_root + "#{Gitlab::Application.config.assets.prefix}/ace")
   var ace_mode = "#{@blob.language.try(:ace_mode)}";
   var editor = ace.edit("editor");
+  editor.setValue("#{escape_javascript(@blob.data)}");
   if (ace_mode) {
     editor.getSession().setMode('ace/mode/' + ace_mode);
   }