Ability for incremental backup jobs to hardlink to existing files in other job

Currently, incremental backup jobs are separate from each other and are backed up to different jetbackup_X_X_XYZ directories. When "account filter" is used with "filter package" option, then when user upgrades package, a fresh backup is created in separate jetbackup_X_X_XYZ directory and is not hardlinked to previous copy, thus duplicating space used on backup server. When user now creates backup on demand, a separate backup job is used, so a new copy is created in another jetbackup_X_X_XYZ directory - space used on backup server is now tripled. In our case, when user has 150 GB of space used, it grows to 450 GB usage on backup server.

Problem is more visible due to https://request.jetapps.com/topic/569-jetbackup-5-should-fall-back-to-original_package-in-directadmin - when we add 10GB temporarily to an account in DirectAdmin, then user package changes to "custom" and doubles space used on backup.

We have noticed increased space usage after moving from backup system also based on rsync, but not fragile to user packages changes. It would be good to have optional ability for backup jobs to hardlink to files from other jobs (as long as same destination is used), so that space used on backup server would not double or triple when user package changes or backup on demand is created.