Running "git pack-objects --no-index-version" will segfault, since the
callback is not prepared to handle the "unset" flag.
In theory this might be used to counteract an earlier "--index-version",
or override a pack.indexversion config setting. But the semantics aren't
immediately obvious, and it's unlikely anybody wants this. Let's just
disable the broken option for now.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>