Re: [PATCH 17/17] midx: enable reachability bitmaps during MIDX compaction

> Enable callers to generate reachability bitmaps when performing MIDX
> layer compaction by combining all existing bitmaps from the compacted
> layers.
> 
> Note that the because of the object/pack ordering described by the

> previous commit, the pseudo-pack order for the compacted MIDX is the
> same as concatenating the individual pseudo-pack orderings for each
> layer in the compaction range.
> 
> As a result, the only non-test or documentation change necessary is to
> treat all objects as non-preferred during compaction so as not to
> disturb the object ordering.
> 
> In the future, we may want to adjust which commit(s) receive
> reachability bitmaps when compacting multiple .bitmap files into one, or
> even generate new bitmaps (e.g., if the references have moved
> significantly since the .bitmap was generated). This commit only
> implements combining all existing bitmaps in range together in order to
> demonstrate and lay the groundwork for more exotic strategies.

> @@ -216,6 +216,8 @@ static int cmd_multi_pack_index_compact(int argc, const char **argv,
>  
>  	struct option *options;
>  	static struct option builtin_multi_pack_index_compact_options[] = {
> +		OPT_BIT(0, "bitmap", &opts.flags, N_("write multi-pack bitmap"),
> +			MIDX_WRITE_BITMAP | MIDX_WRITE_REV_INDEX),
>  		OPT_BIT(0, "incremental", &opts.flags,
>  			N_("write a new incremental MIDX"), MIDX_WRITE_INCREMENTAL),
>  		OPT_END(),