Only show manual merge instructions if current user can merge

app/views/projects/merge_requests/_show.html.haml

@@ -34,8 +34,8 @@
     = render "projects/merge_requests/show/how_to_merge"
     = render "projects/merge_requests/widget/show.html.haml"
 
-    - if @merge_request.open? && @merge_request.can_be_merged?
-      .light.append-bottom-20
+    - if @merge_request.open? && @merge_request.source_branch_exists? && @merge_request.can_be_merged? && @merge_request.can_be_merged_by?(current_user)
+      .light.prepend-top-default
         You can also accept this merge request manually using the
         = succeed '.' do
           = link_to "command line", "#modal_merge_info", class: "how_to_merge_link vlink", title: "How To Merge", "data-toggle" => "modal"