'pushdefault' doesn't come up. This is because "$cur" is matched with
"remote.*" and a list of remotes are completed. Add 'pushdefault' as a
candidate for completion too, using __gitcomp_nl_append ().
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>