pack-bitmap: allow aborting iteration of bitmapped objects

In a subsequent commit we'll lift iteration of bitmapped objects into the "packed" backend and make it accessible via `odb_for_each_object()`. The calling convention for that function is that the callback may return a non-zero exit code, and if so we'll abort iteration. This is currently impossible to realize though, as `for_each_bitmapped_object()` will ignore any return value and just churn through all objects completely. This doesn't matter to the callers of `for_each_bitmapped_object()`, as there's only one of them in git-cat-file(1), and the callbacks we pass always return zero. But once we move the logic into the generic infrastructure it becomes a latent bug waiting to happen. Refactor the code so that the return value of the `show_reach` callback is not ignored anymore. Instead, returning a non-zero value will cause us to abort iteration in both `show_objects_for_type()` and in `for_each_bitmapped_object()`. Note though that there's a second user of `show_objects_for_type()` with `traverse_bitmap_commit_list()`, and that function does indeed invoke callbacks that may return non-zero. This non-zero return value never had any effect at all though, and the callbacks that return non-zero values are only ever invoked via `traverse_bitmap_commit_list()`. Consequently, we adapt them to always return 0. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>