mariabackup prepare fails for incrementals if a new schema is created after full backup is taken

If a new schema with at least one table is created after a full backup is taken, but before the incremental is taken, the backup cannot be restored because incremental prepare fails.

mariabackup prepare fails with : Error on rename ..... (errno: 20 "Not a directory" )

The issue seems to be that mariabackup prepare expects the folder for the new schema to be already there, but it isn't because the schema is created after the full is taken, so can't be found in the full backup.

This is critical as it breaks backup restores in presence of incrementals when the above condition is met (which is all but rare).

How to repeat:

1. take a full backup
2. create a new schema and table
3. take an incremental backup
4. restore the full backup and then the incremental backup; prepare will crash