Speed up syncing of modified versions of existing files. 

One simple way is to find the key of the old version of a file that's
being transferred, so it can be used as the basis for rsync, or any
other similar transfer protocol.

For remotes that don't use rsync, a poor man's version could be had by
chunking each object into multiple parts. Only modified parts need be
transferred. Sort of sub-keys to the main key being stored.
