patchcommit.h: rearrange 'index' to shrink struct commit

> On linux 64-bit architecture, pahole finds that there's a 4 bytes
> padding after 'index'. Moving it to the end reduces this struct's size
> from 72 to 64 bytes (because of another 4 bytes padding after
> graph_pos). On linux 32-bit, the struct size remains 52 bytes like
> before.
>
> Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
> ---
>  commit.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/commit.h b/commit.h
> index e57ae4b583..fd1cbe7263 100644
> --- a/commit.h
> +++ b/commit.h
> @@ -19,11 +19,11 @@ struct commit_list {
>  struct commit {
>  	struct object object;
>  	void *util;
> -	unsigned int index;
>  	timestamp_t date;
>  	struct commit_list *parents;
>  	struct tree *tree;
>  	uint32_t graph_pos;
> +	unsigned int index;
>  };
>  
>  extern int save_commit_buffer;