Good progress on `git annex export` today. Changing the exported tree now
works and is done efficiently. Resuming an export is working. Even
detecting and resolving export conflicts should work (have not tested it).
The necessary information about the export is recorded in the git-annex
branch, including grafting in the exported tree there.

There are some known problems when the tree that is exported contains
multiple files with the same content. And git-annex is not yet able
to download exported files from a special remote. Handling both of those
needs way to get from keys to exported filenames. So, I plan to
populate a sqlite database with that information next.
