Finally got the remote tracking branch for import from S3 into a good shape.

Rather than coaxing git into generating the same commits each time for
imports (which would have needed commit dates to be stable somehow), 
I made git-annex always preserve a reference to the last import commit.

[[s3rename.png]]

Here's how it looks when a rename was exported to S3, which resulted in a
history in S3 that diverged and reconverged with the git history.

[[s3history.png]]

And here's how the history develops as more changes get exported and
imported.

With adb and S3 import done, the first phase of [[todo/import_tree]] is
complete.
