fixed a bug in script testMultipleFullIncrDiffBackupDirStructure

Change-Id: Ic2d81fdee6bac4aebc4b99003df28ff0404b2bae
Reviewed-on: http://review.couchbase.org/37965
Reviewed-by: Ashvinder Singh <ashvinder@couchbase.com>
Tested-by: Ashvinder Singh <ashvinder@couchbase.com>