Restore full backup only when necessary

[ddebeau]

When restoring an incremental backup we shouldn't restore the full backup if the snapshot used for the full backup still exists on the system.

zfs_uploader/zfs_uploader/job.py

Lines 126 to 130 in fd89459

	elif backup_type == 'inc':
	# restore full backup first
	backup_full = self._backup_db.get_backup(backup.dependency)
	self._restore_snapshot(backup_full)
	self._restore_snapshot(backup)