serve restic: use ListR (--fast-list) if available
serve restic: use ListR (--fast-list) if available

For Restic's use case, --fast-list will use less transactions and
calling ListR directly means we can avoid the usual memory overhead.

yonjah/rclone,ncw/rclone,ncw/rclone,yonjah/rclone,olihey/rclone,B4dM4n/rclone,seuffert/rclone,olihey/rclone,seuffert/rclone,seuffert/rclone,B4dM4n/rclone,B4dM4n/rclone,ncw/rclone,olihey/rclone,yonjah/rclone,seuffert/rclone,ncw/rclone,yonjah/rclone,B4dM4n/rclone,olihey/rclone