checkout-index: fix negations of even numbers of -n

checkout-index: fix negations of even numbers of -n

The --no-create was parsed with OPT_BOOLEAN, which has a counting up
logic implemented. Since b04ba2bb (parse-options: deprecate OPT_BOOLEAN,
2011-09-27) the OPT_BOOLEAN is deprecated and is only a define:
	/* Deprecated synonym */
	#define OPTION_BOOLEAN OPTION_COUNTUP

However the variable not_new, which can be counted up by giving
--no-create multiple times, is used to set a bit in the struct checkout
bitfield (defined in cache.h:969, declared at builtin/checkout-index.c:19):

	state.not_new = not_new;

When assigning a value other than 0 or 1 to a bit, all leading digits but
the last are ignored and only the last bit is used for setting the bit
variable.

Hence the following:
	# in git.git:
	$ git status
	# working directory clean
	rm COPYING
	$ git status
	# deleted:    COPYING
	$ git checkout-index -a -n
	$ git status
	# deleted:    COPYING
	# which is expected as we're telling git to not restore or create
	# files, however:
	$ git checkout-index -a -n -n
	$ git status
	# working directory clean, COPYING is restored again!
	# That's the bug, we're fixing here.

By restraining the variable not_new to a value being definitely 0 or 1
by the macro OPT_BOOL the bug is fixed.

Signed-off-by: Stefan Beller <stefanbeller@googlemail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>