github: support only list private affiliated repositories

Created by: unknwon

We currently list all affiliated repositories (includes both private and public) for a given user when fetching user permissions, this is definitely a waste as we only care about private ones.

• GitHub API supports filter repositories by visibility: https://developer.github.com/v3/repos/#list-repositories-for-the-authenticated-user
• The client method needs to accept an additional argument to indicate visibility: https://sourcegraph.com/github.com/sourcegraph/sourcegraph/-/blob/internal/extsvc/github/repos.go?#L453:34