I've been working on matching up the git history with the history from a 
versioned S3 export. Got sidetracked for quite a while building an
efficient way to get the git history up to a certian depth (including all
sides of merge commits) without reading the entire `git log` output.

The history matching is mostly working now, but there's a problem when a
rename is exported to S3, because it's non-atomic on S3 and atomic in git,
and so the histories stop matching up. This is not fatal, just results
in an ugly git history with the right tree at the top of it. It's
not entirely wrong; the git repo and the S3 bucket did legitimately diverge
for a while, so shouldn't the merged history reflect that? The problem is
just that the divergence is not represented in the opimal way.

I hate giving up at the final hurdle, but I feel I need to think about this
some more, so merging `import-from-s3` is postponed for another day, or
likely until Monday.
