Load merge request versions without loading whole diff from database

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

app/models/merge_request_diff.rb

@@ -30,6 +30,10 @@ class MergeRequestDiff < ActiveRecord::Base
   # It allows you to override variables like head_commit_sha before getting diff.
   after_create :save_git_content, unless: :importing?
 
+  def self.select_without_diff
+    select(column_names - ['st_diffs'])
+  end
+
   # Collect information about commits and diff from repository
   # and save it to the database as serialized data
   def save_git_content

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

-- diffs_count = @merge_request.merge_request_diffs.count
+- merge_request_diffs = @merge_request.merge_request_diffs.select_without_diff
+- latest_diff = merge_request_diffs.first
 
-- if diffs_count > 1
-  - latest_diff = @merge_request.merge_request_diff
+- if merge_request_diffs.size > 1
   .mr-version-switch
     Version: 
     %span.dropdown.inline
@@ -13,7 +13,7 @@
             #{@merge_request_diff.head_commit.short_id}
         %span.caret
       %ul.dropdown-menu.dropdown-menu-selectable
-        - @merge_request.merge_request_diffs.each do |merge_request_diff|
+        - merge_request_diffs.each do |merge_request_diff|
           %li
             = link_to diffs_namespace_project_merge_request_path(@project.namespace, @project, @merge_request, diff_id: merge_request_diff.id), class: ('is-active' if merge_request_diff == @merge_request_diff) do
               %strong.monospace