'Missing author' note is being added to correctly mapped users on Project/Group Import

When using Project/Group Import existing users on destination GitLab instance can be mapped to exported relations from a project/group (when import is being done by an admin). If a user is not mapped, any content will be author by an importer user and for each note there will be an additional text describing the original author, for example:

Source:

Destination:

However, when user on destination exists and was properly mapped, a missing author note By George Koltsov on 2020-03-24T12:06:33 (imported from GitLab project) is still being added, when it shouldn't.

From #19128 (comment 309051263)

It looks like old_author_id https://gitlab.com/gitlab-org/gitlab/-/blob/v12.8.7-ee/lib/gitlab/import_export/base_relation_factory.rb#L242 is not a source user id, it's a destination user id, since it gets replaced in update_user_references before this method is executed https://gitlab.com/gitlab-org/gitlab/blob/master/lib/gitlab/import_export/base/relation_factory.rb#L115-115