HTTP API to fetch file from repository following symlinks

In bug 1797366 we have a requirement to fetch a file from GitHub following symlinks. In the process of working out how to fix this I see that we have the same kind of problem with Launchpad's own Git service; while cgit has <repository>/plain/<file> URLs, those don't follow symlinks (GitHub returns the symlink target path, while cgit just 404s).

It'd be useful for turnip to provide URLs that fetch a file while following repository-internal symlinks, perhaps with some guards against excessive symlink depth.