Fix migration corrupting import data for old version upgrades

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/19073

Not a fan of changing old migrations but this will prevent issues from upgrading GitLab version 8.6 or older to 8.10

More details of the issue on https://gitlab.com/gitlab-org/gitlab-ce/issues/19073#note_13190450

See merge request !5358