Improve performance of commits endpoint

Currently all the commits of a merge request is loaded
into the memory, converted to CommitCollection and
then paginated as an array

We can dramatically improve the performance by
paginating the diff commits first and then convert
this smaller array into CommitCollection

lib/api/merge_requests.rb

@@ -296,9 +296,12 @@ module API
       end
       get ':id/merge_requests/:merge_request_iid/commits' do
         merge_request = find_merge_request_with_access(params[:merge_request_iid])
-        commits = ::Kaminari.paginate_array(merge_request.commits)
 
-        present paginate(commits), with: Entities::Commit
+        commits =
+          paginate(merge_request.merge_request_diff.merge_request_diff_commits)
+            .map { |commit| Commit.from_hash(commit.to_hash, merge_request.project) }
+
+        present commits, with: Entities::Commit
       end
 
       desc 'Show the merge request changes' do