Re: [PATCH v2 01/15] pack-bitmap: factor out type iterator initialization

> When count_object_type() wants to iterate over the bitmap of all objects
> of a certain type, we have to pair up OBJ_COMMIT with bitmap->commits,
> and so forth. Since we're about to add more code to iterate over these
> bitmaps, let's pull the initialization into its own function.
>
> We can also use this to simplify traverse_bitmap_commit_list(). It
> accomplishes the same thing by manually passing the object type and the
> bitmap to show_objects_for_type(), but using our helper we just need the
> object type.
>
> Note there's one small code change here: previously we'd simply return
> zero when counting an unknown object type, and now we'll BUG(). This
> shouldn't matter in practice, as all of the callers pass in only usual
> commit/tree/blob/tag types.