Order only for latest_successful_for

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8286#note_20461082

app/models/ci/pipeline.rb

@@ -93,11 +93,8 @@ module Ci
         .select("max(#{quoted_table_name}.id)")
         .group(:ref, :sha)
 
-      if ref
-        where(id: max_id, ref: ref)
-      else
-        where(id: max_id)
-      end
+      relation = ref ? where(ref: ref) : self
+      relation.where(id: max_id)
     end
 
     def self.latest_status(ref = nil)
@@ -105,7 +102,7 @@ module Ci
     end
 
     def self.latest_successful_for(ref)
-      success.latest(ref).first
+      success.latest(ref).order(id: :desc).first
     end
 
     def self.truncate_sha(sha)