Stop overwriting the backup when creating incremental backups

Release notes

Problem to solve

When creating an incremental repository backup you need to specify which backup will be used to create a differential from. This is done using the BACKUP env var. Once specified, this backup is untar'd into the backup staging area where the incremental repository backup can be written.

Once this is completed, the same BACKUP var is used to re-create the backup tar file, overwriting the original backup.

This may be causing issues with backup file pruning since no newly named archive is created https://docs.gitlab.com/ee/raketasks/backup_restore.html#limit-backup-lifetime-for-local-files-prune-old-backups

Proposal

Use the newly introduced INCREMENTAL variable to specify which backup to unpack to create the differential from.

For example:

sudo gitlab-backup create BACKUP=new_backup INCREMENTAL=previous_backup

In order to maintain compatibility with INCREMENTAL=on we should only use this var to unpack a backup if it doesn't parse as a boolean. The following will still unpack and overwrite new_backup:

sudo gitlab-backup create BACKUP=new_backup INCREMENTAL=on

For gitaly-backup to maintain its own filesystem it needs to know which backup was the original full backup. This original full backup name will need to be stored within backup_information.yml for this purpose.

Intended users

Feature Usage Metrics

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