rdiff-backup changes permissions of source directory

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http://unix.stackexchange.com – I recently found rdiff-backup which seems to be very useful incremental alternative to rsync. But there's an issue, while trying to use the rdiff-backup to make a mirror of a folder in other one, it changes permissions of source folder ( specially it changes the read-only 400 to 644 ) Any suggestion on how to fix it? (HowTos)