Allow to pull contents without credentials on public repositories

[mika-f]

Normally, we can pull public repositories even if we are not a GitHub user, and the same goes for content managed by Git LFS.
However, the current git-lfs-proxy requires authentication even for public repositories, and will fail if we are an unauthenticated user or use open CI.
Therefore, if it is a Public Repository (= if the repository can be retrieved without octokit authentication), authentication information is not required only for Pull.