Compare to Previous URL not updated with external_url change

Summary

When the external_url is changed in the gitlab.rb config file, most links in the Web interface are updated as well.

In a Merge Request, the Compare to previous versions url is not updated and retains the original external_url

Steps to reproduce

1. Open merge request
2. Push revision to merge request branch
3. Note that the Compare to previous version url is relative to the current external_url
4. modify external_url and reconfigure
5. Note that the Compare to previous version url is relative to the previous external_url

What is the expected correct behavior?

This url should be relative to the current external_url after the URL has been changed.