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