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

The parsing of "--index" is done in a callback, but it does
not handle an "unset" option. We don't necessarily expect
anyone to use this, but the current behavior is to treat it
exactly like "--index", which would probably be surprising.

Instead, let's just turn it into an OPT_BOOL, and handle it
after we're done parsing. This makes "--no-index" just work
(it cancels a previous "--index").

As a bonus, this makes the logic easier to follow. The old
code opened the index during the option parsing, leaving the
reader to wonder if there was some timing issue (there
isn't; none of the other options care that we've opened it).
And then if we found that "--prefix" had been given, we had
to rollback the index. Now we can simply avoid opening it in
the first place.

Note that it might make more sense for checkout-index to
complain when "--index --prefix=foo" is given (rather than
silently ignoring "--index"), but since it has been that way
since 415e96c ([PATCH] Implement git-checkout-cache -u to
update stat information in the cache., 2005-05-15), it's
safer to leave it as-is.

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

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