bitmap: add bitmap_unset() function

bitmap: add bitmap_unset() function

We've never needed to unset an individual bit in a bitmap until now.
Typically they start with all bits unset and we bitmap_set() them, or we
are applying another bitmap as a mask. But the easiest way to apply an
object filter to a bitmap result will be to unset the individual bits.

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