Improve performance for merge_request#show page

* store @participants in variable
* store result of subscribed? call into variable

In total it reduce amount of SQL queries for merge_request with 10
comments/participants twice.
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/controllers/projects/merge_requests_controller.rb

@@ -246,6 +246,8 @@ class Projects::MergeRequestsController < Projects::ApplicationController
   end
 
   def define_show_vars
+    @participants = @merge_request.participants(current_user, @project)
+
     # Build a note object for comment form
     @note = @project.notes.new(noteable: @merge_request)
     @notes = @merge_request.mr_and_commit_notes.inc_author.fresh

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

@@ -30,18 +30,19 @@
         = f.submit class: 'btn'
 
   - if current_user
+    - subscribed = @merge_request.subscribed?(current_user)
     %div.prepend-top-20.clearfix
       .issuable-context-title
         %label
           Subscription:
       %button.btn.btn-block.subscribe-button{:type => 'button'}
         = icon('eye')
-        %span= @merge_request.subscribed?(current_user) ? 'Unsubscribe' : 'Subscribe'
-      - subscribtion_status = @merge_request.subscribed?(current_user) ? 'subscribed' : 'unsubscribed'
+        %span= subscribed ? 'Unsubscribe' : 'Subscribe'
+      - subscribtion_status = subscribed ? 'subscribed' : 'unsubscribed'
       .subscription-status{data: {status: subscribtion_status}}
-        .description-block.unsubscribed{class: ( 'hidden' if @merge_request.subscribed?(current_user) )}
+        .description-block.unsubscribed{class: ( 'hidden' if subscribed )}
           You're not receiving notifications from this thread.
-        .description-block.subscribed{class: ( 'hidden' unless @merge_request.subscribed?(current_user) )}
+        .description-block.subscribed{class: ( 'hidden' unless subscribed )}
           You're receiving notifications because you're subscribed to this thread.
 
 :coffeescript

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

 .participants
-  %span #{@merge_request.participants(current_user).count} participants
-  - @merge_request.participants(current_user).each do |participant|
+  %span #{@participants.count} participants
+  - @participants.each do |participant|
     = link_to_member(@project, participant, name: false, size: 24)