Fix migrate_in not working within a transaction

Co-Authored-By: Yorick Peterse <yorick@yorickpeterse.com>

db/post_migrate/20210901153324_slice_merge_request_diff_commit_migrations.rb

@@ -3,6 +3,8 @@
 class SliceMergeRequestDiffCommitMigrations < ActiveRecord::Migration[6.1]
   include Gitlab::Database::MigrationHelpers
 
+  disable_ddl_transaction!
+
   BATCH_SIZE = 5_000
   MIGRATION_CLASS = 'MigrateMergeRequestDiffCommitUsers'
   STEAL_MIGRATION_CLASS = 'StealMigrateMergeRequestDiffCommitUsers'
@@ -15,6 +17,7 @@ class SliceMergeRequestDiffCommitMigrations < ActiveRecord::Migration[6.1]
 
     return if old_jobs.empty?
 
+    transaction do
       # This ensures we stop processing the old ranges, as the background
       # migrations skip already processed jobs.
       Gitlab::Database::BackgroundMigrationJob
@@ -40,6 +43,7 @@ class SliceMergeRequestDiffCommitMigrations < ActiveRecord::Migration[6.1]
       end
 
       Gitlab::Database::BackgroundMigrationJob.insert_all!(rows)
+    end
 
     job = Gitlab::Database::BackgroundMigrationJob
       .for_migration_class(MIGRATION_CLASS)