pack-bitmap-write: rebuild using new bitmap when remapping

pack-bitmap-write: rebuild using new bitmap when remapping

`git repack` is about to learn a new `--filter=<filter-spec>` option and
we will want to check that this option is incompatible with
`--write-bitmap-index`.

Unfortunately it appears that a test like:

test_expect_success '--filter fails with --write-bitmap-index' '
       test_must_fail \
               env GIT_TEST_MULTI_PACK_INDEX_WRITE_BITMAP=0 \
               git -C bare.git repack -a -d --write-bitmap-index --filter=blob:none
'

sometimes fail because when rebuilding bitmaps, it appears that we are
reusing existing bitmap information. So instead of detecting that some
objects are missing and erroring out as it should, the
`git repack --write-bitmap-index --filter=...` command succeeds.

Let's fix that by making sure we rebuild bitmaps using new bitmaps
instead of existing ones.

Helped-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>