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 <696e3fbcf235d40b6ea1ebd61c87cbea79d444e7@peff.net>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git