Ensure all refs are restored on a restore from backup

To restore all refs from a bundle, we need to use the --mirror option to
ensure that all refs are properly restored. Without this, refs such as
`refs/merge-requests/X` and `refs/keep-around/Y` would be omitted, which
exposed a bug as described in https://gitlab.com/gitlab-org/gitlab-ce/issues/38319#note_41648608.

Closes #38352

changelogs/unreleased/sh-restore-all-refs-backup.yml

+---
+title: Ensure all refs are restored on a restore from backup
+merge_request:
+author:
+type: fixed

lib/backup/repository.rb

@@ -78,7 +78,7 @@ module Backup
         project.ensure_storage_path_exists
 
         cmd = if File.exist?(path_to_project_bundle)
-                %W(#{Gitlab.config.git.bin_path} clone --bare #{path_to_project_bundle} #{path_to_project_repo})
+                %W(#{Gitlab.config.git.bin_path} clone --bare --mirror #{path_to_project_bundle} #{path_to_project_repo})
               else
                 %W(#{Gitlab.config.git.bin_path} init --bare #{path_to_project_repo})
               end