checkout-index: handle "--no-prefix" option

checkout-index: handle "--no-prefix" option

We use a custom callback to parse "--prefix", but it does
not handle the "unset" case. As a result, passing
"--no-prefix" will cause a segfault.

We can fix this by switching it to an OPT_STRING, which
makes "--no-prefix" counteract a previous "--prefix". Note
that this assigns NULL, so we bump our default-case
initialization to lower in the main function.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>