What are the zip files kept under /var/opt/gitlab/gitlab-rails/tmp/repositories/<reponame>?

I was alerted that the /var partition on our gitlab server had almost filled up. I noticed that moments before a number of large zip file had been dumped in the /var/opt/gitlab/gitlab-rails/tmp/repositories/ directory. What are these files? Is it safe to get rid of them? What is the likely cause of the error?

In the production.log at around the same time I found the following.

Completed 500 Internal Server Error in 42450ms

RuntimeError (git --git-dir=/var/opt/gitlab/git-data/repositories//.git archive --prefix=.git/ --format=zip – master failed):
app/models/repository.rb:169:in method_missing' app/services/archive_repository_service.rb:12:in execute’
app/controllers/projects/repositories_controller.rb:18:in `archive’