pack-bitmap-write.c: gracefully fail to write non-closed bitmaps

pack-bitmap-write.c: gracefully fail to write non-closed bitmaps

The set of objects covered by a bitmap must be closed under
reachability, since it must be the case that there is a valid bit
position assigned for every possible reachable object (otherwise the
bitmaps would be incomplete).

Pack bitmaps are never written from 'git repack' unless repacking
all-into-one, and so we never write non-closed bitmaps (except in the
case of partial clones where we aren't guaranteed to have all objects).

But multi-pack bitmaps change this, since it isn't known whether the
set of objects in the MIDX is closed under reachability until walking
them. Plumb through a bit that is set when a reachable object isn't
found.

As soon as a reachable object isn't found in the set of objects to
include in the bitmap, bitmap_writer_build() knows that the set is not
closed, and so it now fails gracefully.

A test is added in t0410 to trigger a bitmap write without full
reachability closure by removing local copies of some reachable objects
from a promisor remote.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>