+dit(bf(--compare-dest DIR)) This option instructs rsync to use DIR as an
+additional directory to compare destination files against when doing
+transfers. This is useful for doing transfers to a new destination while
+leaving existing files intact, and then doing a flash-cutover when all
+files have been successfully transfered (for example by moving directories
+around and removing the old directory). This option increases the
+usefulness of --partial because partially transferred files will remain in
+the new temporary destination until they have a chance to be completed.
+If DIR is a relative path, it is relative to the destination directory.
+