Git push SSH proxy may not handle relative_url_root

I ran into this during another MR.

If external_url is http://example.com/gitlab, it looks like https://gitlab.com/gitlab-org/gitlab/-/blob/master/ee/lib/gitlab/geo/git_ssh_proxy.rb#L142 puts the relative_url_root into the auth_scope.

def auth_scope
  URI.parse(primary_repo).path.gsub(%r{^\/|\.git$}, '')
end