Migration to write fullpath in all repository configs

What does this MR do?

In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16027 it was added to write gitlab.fullpath in the git config of all repositories. But this only writes them on move or migrate to hashed storage.

This adds a migration that writes the fullpath to all the repositories.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/41776

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