Hyperlink support for submodules with relative paths

Submodules defined with relative paths are now clickable in the web UI, making it easy to navigate to linked repositories. Previously, only submodules with absolute URLs were clickable.

Only relative paths following the format ../{repo} (a repository with the same owner) or ../{owner}/{repo} (a repository with a different owner) are supported.

To learn more about submodules, see Working with submodules on the GitHub blog.