WebEditor: Check if save is possible

app/controllers/tree_controller.rb

@@ -25,9 +25,8 @@ class TreeController < ProjectResourceController
   end
 
   def update
-    last_commit = @project.commits(@ref, @path, 1).first.sha
-    file_editor = Gitlab::FileEditor.new(current_user, @project)
-    if file_editor.can_edit?(@path, last_commit)
+    file_editor = Gitlab::FileEditor.new(current_user, @project, @ref)
+    if file_editor.can_edit?(@path, params[:last_commit])
       file_editor.update(@path, params[:content])
       redirect_to project_tree_path(@project, @id), :notice => "File has been successfully changed"
     else

lib/gitlab/file_editor.rb

 module Gitlab
   class FileEditor
 
-    attr_accessor :user, :project
+    attr_accessor :user, :project, :ref
 
-    def initialize(user, project)
+    def initialize(user, project, ref)
       self.user = user
       self.project = project
+      self.ref = ref
     end
 
     def can_edit?(path, last_commit)
-      true
+      current_last_commit = @project.commits(ref, path, 1).first.sha
+      last_commit == current_last_commit
     end
 
     def update(path, content)