Add `--incremental` command-line option to `barman backup` command. This is used to specify the parent backup when taking an incremental backup. The parent can be either a full backup or another incremental backup.
Add `latest-full` shortcut backup ID. Along with `latest`, this can be used as a shortcut to select the parent backup for an incremental backup. While `latest` takes the latest backup independently if it is full or incremental, `latest-full` takes the latest full backup.
`barman keep` command can only be applied to full backups when `backup_method = postgres`. If a full backup has incremental backups that depend on it, all of the incrementals are also kept by Barman.
Retention policies do not take incremental backups into consideration. As incremental backups cannot be recovered without having the complete chain of backups available up to the full backup, only full backups account for retention policies.
`barman recover` needs to combine the full backup with the chain of incremental backups when recovering. The new CLI option `--local-staging-path`, and the corresponding `local_staging_path` configuration option, are used to specify the path in the Barman host where the backups will be combined when recovering an incremental backup.