pack-bitmap: implement BLOB_NONE filtering

We can easily support BLOB_NONE filters with bitmaps. Since we know the
types of all of the objects, we just need to clear the result bits of
any blobs.

Note two subtleties in the implementation (which I also called out in
comments):

  - we have to include any blobs that were specifically asked for (and
    not reached through graph traversal) to match the non-bitmap version

  - we have to handle in-pack and "ext_index" objects separately.
    Arguably prepare_bitmap_walk() could be adding these ext_index
    objects to the type bitmaps. But it doesn't for now, so let's match
    the rest of the bitmap code here (it probably wouldn't be an
    efficiency improvement to do so since the cost of extending those
    bitmaps is about the same as our loop here, but it might make the
    code a bit simpler).

Here are perf results for the new test on git.git:

  Test                                    HEAD^             HEAD
  --------------------------------------------------------------------------------
  5310.9: rev-list count with blob:none   1.67(1.62+0.05)   0.22(0.21+0.02) -86.8%

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

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