MFC r257879:

Fix typo in r256646: We want to generate lists of directories in
INDEX-OLD and INDEX-NEW and compare them, not generate the same
list of directories from INDEX-OLD twice...

Pointy hats to: cperciva & everybody who didn't proofread
		EN-13:04 enough
Errata Notice:	FreeBSD-EN-13:05.freebsd-update
Approved by:	so
Merge branch 'releng/9.1' into feature/blocksize/releng/9.1