If git clone is given more than two non-option arguments, it
silently throws away all but the first one.  Complain instead.

Discovered by comparing the new builtin clone to the old
git-clone.sh.
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>