Support Git remote URL and auth parameters directly for fetch_remote

To support fetches for Geo, repository imports and mirroring, we write out the gitconfig directly to the repository, but once Gitaly is complete, we won't have access to do this.

We should pass the required data directly to the fetch_remote call, as we currently do for SSH authentication.

Requisite pieces of data:

• Remote URL (may contain credentials)
• Additional HTTP header, if any (may contain credentials, required for Geo, needs Git >= 2.9.0)
• SSH authentication (works well)

Overall, I think we can stop writing remotes to .gitconfig entirely, replacing them with the remote URL. For gitlab-shell that would have to go into an envvar, since it may contain credentials.