Fix git submodule link for subgroup projects with relative path

What does this MR do?

Use Ruby build-in File#absolute_path to resolve git submodule link to projects on GitLab.com using relative path.

On the web interface, when a repo references a submodule with relative path, it creates a link so user to jump to the submodule repo easily. However this was implemented before we had subgroup, so the subgroup support was buggy. A proper relative path resolving logic is added to handle this.

What are the relevant issue numbers?

Fix #37356 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

Closes #37356 (closed)