pack-bitmap.c: harden 'test_bitmap_walk()' to check type bitmaps

pack-bitmap.c: harden 'test_bitmap_walk()' to check type bitmaps

The special `--test-bitmap` mode of `git rev-list` is used to compare
the result of an object traversal with a bitmap to check its integrity.
This mode does not, however, assert that the types of reachable objects
are stored correctly.

Harden this mode by teaching it to also check that each time an object's
bit is marked, the corresponding bit should be set in exactly one of the
type bitmaps (whose type matches the object's true type).

Co-authored-by: Jeff King <peff@peff.net>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>