Allow curl helper to work without a local repository

Allow curl helper to work without a local repository

It's okay to use the curl helper without a local repository, so long
as you don't use "fetch". There aren't any git programs that would try
to use it, and it doesn't make sense to try it (since there's nowhere
to write the results), but we may as well be clear.

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>