Fix IDE get file data with '/' as relative root

What does this MR do?

A recent update in gdk caused relative_url_root to default in / which causes a number of issues. This MR fixes the Web IDE files not being fetched.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Links

• Related issue https://gitlab.com/gitlab-org/gitlab-ce/issues/60932