Work around a bug in Rails 5, where LIMIT causes trouble

The original code caused Rails to generate invalid SQL. The problem
lays in the `.arel` method in `ActiveRecord::Relation`. When there was
a `limit` on the relation, the `LIMIT` statement was taken over to
Arel, but the value wasn't.

```ruby
relation = Event.limit(2)
relation.to_sql
#=> "SELECT  `events`.* FROM `events` LIMIT 2"
relation.arel.to_sql
#=> "SELECT  `events`.* FROM `events` LIMIT ?"
```

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/51729

lib/gitlab/database/subquery.rb

@@ -6,7 +6,15 @@ module Gitlab
       class << self
         def self_join(relation)
           t = relation.arel_table
-          t2 = relation.arel.as('t2')
+          t2 = if !Gitlab.rails5?
+                 relation.arel.as('t2')
+               else
+                 # Work around a bug in Rails 5, where LIMIT causes trouble
+                 # See https://gitlab.com/gitlab-org/gitlab-ce/issues/51729
+                 r = relation.limit(nil).arel
+                 r.take(relation.limit_value) if relation.limit_value
+                 r.as('t2')
+               end
 
           relation.unscoped.joins(t.join(t2).on(t[:id].eq(t2[:id])).join_sources.first)
         end