patchfix an 'dubious one-bit signed bitfield' error

> 
> Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
> ---
> 
> Hi Jeff,
> 
> If you need to re-roll your 'jh/object-filtering' branch, could
> you please squash this into the relevant commit (b87fd93d81,
> "list-objects: filter objects in traverse_commit_list", 02-11-2017).
> 
> [This error was issued by sparse]
> 
> Thanks!
> 
> ATB,
> Ramsay Jones
> 
>   list-objects-filter.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/list-objects-filter.c b/list-objects-filter.c
> index 7f2842547..d9e626be8 100644
> --- a/list-objects-filter.c
> +++ b/list-objects-filter.c
> @@ -191,7 +191,7 @@ static void *filter_blobs_limit__init(
>    */
>   struct frame {
>   	int defval;
> -	int child_prov_omit : 1;
> +	unsigned int child_prov_omit : 1;
>   };
>   
>   struct filter_sparse_data {
>