hash-object: handle --literally with OPT_NEGBIT

hash-object: handle --literally with OPT_NEGBIT

Since we recently removed the hash_literally() function, the hash-object
--literally option has been simplified to just removing the
INDEX_FORMAT_CHECK flag. Rather than pass it around as a separate bool,
we can just have the option parser remove the bit from the set of flags
directly. This simplifies the helper functions.

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