I realized writing a git repository conversion tool is actually quite trivial in Perl, so I wrote one up and tried it on the git repo included in the git-0.04 tarball. It fsck-cache's cleanly on both the nonflat and flat base64 patches, at least after I had fixed a minor bug in fsck-cache in the nonflat patch (now uploaded.)
It takes an source and a target object directory as arguments. By default it is set up for non-flat representation; change $flat = 0; to $flat = 1; near the top to use flat representation.
ftp://ftp.kernel.org/pub/linux/kernel/people/hpa/gitcvt
Comments appreciated, of course.
-hpa