Show merge in progress message if MR is locked

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

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

@@ -21,7 +21,7 @@
 
   - content_for :note_actions do
     - if can?(current_user, :modify_merge_request, @merge_request)
-      - unless @merge_request.closed? || @merge_request.merged?
+      - if @merge_request.open?
         = link_to 'Close', project_merge_request_path(@project, @merge_request, merge_request: {state_event: :close }), method: :put, class: "btn btn-grouped btn-close close-mr-link js-note-target-close", title: "Close merge request"
       - if @merge_request.closed?
         = link_to 'Reopen', project_merge_request_path(@project, @merge_request, merge_request: {state_event: :reopen }), method: :put, class: "btn btn-grouped btn-reopen reopen-mr-link js-note-target-reopen", title: "Reopen merge request"

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

@@ -21,6 +21,12 @@
         #{time_ago_with_tooltip(@merge_request.merge_event.created_at)}
       = render "projects/merge_requests/show/remove_source_branch"
 
+    - if @merge_request.locked?
+      %h4
+        Merge in progress...
+      %p
+        GitLab tries to merge it right now. During this time merge request is locked and can not be closed.
+
     - unless @commits.any?
       %h4 Nothing to merge
       %p