Relative URL (external_url) is not respected with ?go-get=1 after upgrading to 17.5.2

Summary

A US Government customer (Access required) is reporting that when they have set external_url with a relative url in their configuration, the URL isn't respected when using the ?go-get=1 option.

Example: https://example.com/gitlab/group/project?go-get=1

The expected page should show:

go get example.com/gitlab/group/project

but instead shows:

example.com/group/project

and therefore is not respecting the relative URL setting as expected.

Steps to reproduce

• Start with a version <17.5.2 (Customer case is 17.2.9 -> 17.3.7 -> 17.5.2 )
• Set external_url 'https://example.com/gitlab' in gitlab.rb
• Confirm ?go-get=1 works on the project
• Upgrade to 17.5.2
• Confirm ?go-get=1 no longer works

Example Project

Unable to reproduce on gitlab.com as external_url cannot be set.

What is the current bug behavior?

?go-get=1 shows the incorrect URL

What is the expected correct behavior?

?go-get=1 shows the correct URL

Relevant logs and/or screenshots

None available at this time.

I will attach env and check rake tasks from the customer when we are able to share them.

Added frontend label per https://docs.gitlab.com/omnibus/settings/configuration.html#relative-url-troubleshooting