Render MR diff correctly

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/controllers/projects/merge_requests_controller.rb

@@ -213,6 +213,7 @@ class Projects::MergeRequestsController < Projects::ApplicationController
     # or from cache if already merged
     @commits = @merge_request.commits
 
+    @merge_request_diff = @merge_request.merge_request_diff
     @allowed_to_merge = allowed_to_merge?
     @show_merge_controls = @merge_request.opened? && @commits.any? && @allowed_to_merge
   end

app/views/projects/merge_requests/show/_diffs.html.haml

-- if @merge_request.valid_diffs?
+- if @merge_request_diff.collected?
   = render "projects/commits/diffs", diffs: @merge_request.diffs, project: @merge_request.source_project
-- elsif @merge_request.broken_diffs?
+- elsif @merge_request_diff.empty?
+  %h4.nothing_here_message Nothing to merge from #{@merge_request.source_branch} into #{@merge_request.target_branch}
+- else
   %h4.nothing_here_message
     Can't load diff.
     You can
     = link_to "download it", project_merge_request_path(@merge_request.source_project, @merge_request), format: :diff, class: "vlink"
     instead.
-- else
-  %h4.nothing_here_message Nothing to merge