Only access ssh_config for hosts other than "github.com"

Assume that "github.com" is not an ssh alias to anywhere else. This
optimizes the most common case in which all remotes in a repository
point to "github.com" in their URLs and in which we can avoid
instantiating SshConfig altogether.

lib/hub/context.rb

@@ -387,7 +387,10 @@ module Hub
 
       def uri_parse uri
         uri = URI.parse uri
-        uri.host = local_repo.ssh_config.get_value(uri.host, :HostName) { uri.host }
+        if uri.host != local_repo.default_host
+          ssh = local_repo.ssh_config
+          uri.host = ssh.get_value(uri.host, :HostName) { uri.host }
+        end
         uri
       end
     end