Partially revert "Optimise the external diff storage migration query"

This solved one problem, but introduced another. Revert the functional
change while keeping the rearrangement of code to make future changes
easier.

app/models/merge_request_diff.rb

@@ -115,15 +115,7 @@ class MergeRequestDiff < ApplicationRecord
     end
 
     def ids_for_external_storage_migration_strategy_always(limit:)
-      ids = []
-
-      files_in_database.each_batch(column: :merge_request_id, order_hint: :id) do |scope|
-        ids.concat(scope.has_diff_files.pluck(:id))
-
-        break if ids.count >= limit
-      end
-
-      ids.first(limit)
+      has_diff_files.files_in_database.limit(limit).pluck(:id)
     end
 
     def ids_for_external_storage_migration_strategy_outdated(limit:)

changelogs/unreleased/227570-fix-merge-request-diff-migrate-timeout.yml

----
-title: Optimise the external diff storage migration query
-merge_request: 38579
-author:
-type: performance