Modify index_has_changes() to take a struct istate* instead of just
operating on the_index.  This is only a partial conversion, though,
because we call do_diff_cache() which implicitly assumes work is to be
done on the_index.  Ongoing work is being done elsewhere to do the
remainder of the conversion, and thus is not duplicated here.  Instead,
a simple check is put in place until that work is complete.
Signed-off-by: NElijah Newren <newren@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>