Error 500 viewing merge request due to nil commit_email_hostname

https://sentry.gitlab.net/gitlab/gitlabcom/issues/580878/

TypeError: no implicit conversion of nil into String
  gitlab/private_commit_email.rb:9:in `escape'
    hostname_regexp = Regexp.escape(Gitlab::CurrentSettings.current_application_settings.commit_email_hostname)
  gitlab/private_commit_email.rb:9:in `regex'
    hostname_regexp = Regexp.escape(Gitlab::CurrentSettings.current_application_settings.commit_email_hostname)
  gitlab/private_commit_email.rb:15:in `user_id_for_email'
    match = email&.match(regex)
  gitlab/private_commit_email.rb:22:in `block in user_ids_for_emails'
    emails.map { |email| user_id_for_email(email) }.compact.uniq
  gitlab/private_commit_email.rb:22:in `map'
    emails.map { |email| user_id_for_email(email) }.compact.uniq
...
(242 additional frame(s) were not displayed)

ActionView::Template::Error: no implicit conversion of nil into String

ActionView::Template::Error: no implicit conversion of nil into String