Oh, does it use mmap exclusively for all file access now? Including repacking and everything? Neat.
You need two local copies if you want to do local version control, and also to let people edit files normally on disk (as opposed to, say, interposing a FUSE filesystem to observe edits as they happen). But the systems we're talking about are not trying to do local version control. They're trying to do remote backup and syncing!
*For this use-case*, you might almost be better off with CVS than with git. Its handling of binary files is dumb, but at least it wouldn't double your local storage requirements. Even better, of course, would be a system that stored the second copy on the remote server only, and then used something clever like rsync to upload the deltas.
Or maybe one could do something clever with libgit2 and librsync to let you directly and efficiently commit a local set of files to a remote bare repository...