`git annex export` can efficiently handle renames, including renames that swap
content between files. However, when there are two pairs of duplicate files,
and the filenames are swapped around, the current rename handling renames both
dups to a single  temp file, and so the other file in the pair gets re-uploaded
unncessarily. This could be improved.

Perhaps: Find pairs of renames that swap content between two files.
Run each pair in turn. Then run the current rename code. Although this
still probably misses cases, where eg, content cycles amoung 3 files, and
the same content amoung 3 other files. Is there a general algorythm?
