Fix branches/compare butten to directly diff to default branch

app/views/projects/branches/_branch.html.haml

@@ -11,7 +11,8 @@
     .pull-right
       - if can?(current_user, :download_code, @project)
         = render 'projects/repositories/download_archive', ref: branch.name, btn_class: 'grouped btn-group-small'
-      = link_to project_compare_index_path(@project, from: branch.name, to: branch.name), class: 'btn grouped btn-small', title: "Compare" do
+      - if branch.name != @repository.root_ref
+        = link_to project_compare_index_path(@project, from: @repository.root_ref, to: branch.name), class: 'btn grouped btn-small', method: :post, title: "Compare" do
           %i.icon-copy
           Compare