Refactor merge request version compare feature

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

app/controllers/projects/merge_requests_controller.rb

@@ -542,7 +542,7 @@ class Projects::MergeRequestsController < Projects::ApplicationController
   end
 
   def compared_diff_version
-    compare = CompareService.new.execute(@project, @merge_request_diff.head_commit_sha, @project, @start_sha)
+    compare = @merge_request_diff.compare_with(@start_sha)
 
     if compare
       @diffs = compare.diffs(diff_options)
@@ -551,11 +551,7 @@ class Projects::MergeRequestsController < Projects::ApplicationController
   end
 
   def original_diff_version
-    unless @merge_request_diff.latest?
-      # Disable comments if browsing older version of the diff
-      @diff_notes_disabled = true
-    end
-
+    @diff_notes_disabled = !@merge_request_diff.latest?
     @diffs = @merge_request_diff.diffs(diff_options)
   end
 end

app/models/merge_request_diff.rb

@@ -152,6 +152,10 @@ class MergeRequestDiff < ActiveRecord::Base
     self == merge_request.merge_request_diff
   end
 
+  def compare_with(sha)
+    CompareService.new.execute(project, head_commit_sha, project, sha)
+  end
+
   private
 
   def dump_commits(commits)

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

@@ -24,7 +24,7 @@
                 = time_ago_with_tooltip(merge_request_diff.created_at)
 
     - if @merge_request_diff.base_commit_sha
-       and 
+      and
       %span.dropdown.inline.mr-version-compare-dropdown
         %a.btn-link.dropdown-toggle{ data: {toggle: :dropdown} }
           %strong
@@ -51,8 +51,10 @@
                 #{@merge_request.target_branch} (base)
               .monospace #{short_sha(@merge_request_diff.base_commit_sha)}
 
-
     - unless @merge_request_diff.latest? && !@start_sha
       .prepend-top-10
         = icon('info-circle')
-        Comments are disabled while viewing outdated merge versions or comparing to versions other than base.
+        - if @start_sha
+          Comments are disabled because you're comparing two versions of this merge request.
+        - else
+          Comments are disabled because you're viewing an old version of this merge request.