LP #1611568: target-dir no longer relative to current directory

**Reported in Launchpad by Ryan Brothers last update 22-11-2016 16:26:54

In 2.4.4, it seems that the --target-dir is no longer relative to the current directory, and instead relative to the --data-dir.

If I'm in the directory /tmp and run:

xtrabackup --backup --datadir=/var/lib/mysql --target-dir=b1

In 2.3.5, the backup is put in /tmp/b1.

In 2.4.4, the backup is put in /var/lib/mysql/b1