parseopt: add OPT_NEGBIT

On Thu, May 07, 2009 at 09:44:17PM +0200, René Scharfe wrote:
Subject: [PATCH] ls-files: make --no-empty-directory properly negatable

This option was specified to parseopt as an OPT_BIT; however, we
actually want to _set_ the bit on --no-empty-directory. Thus the
existing implementation used --no-empty-directory, and required
--no-no-empty-directory to negate it.

Now that OPT_NEGBIT exists, we can properly support it as
--empty-directory and --no-empty-directory (but of course
still defaulting to showing empty directories).

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