Feature/PR proposal: Merge incremental backups into full (synthetic full backup)

I don't know if this is possible at all, but it would be nice if incrementals could be merged back into a full backup.

For our use case, a month of backups is enough. Being able to have a rolling full backup where the oldest incremental is merged with the full would save on disk space for two months of data.

Again, not something we require, but it would be cool.