The ?xmpp screencast is at long last done!

Fixed a bug that could cause the assistant to unstage files from git sometimes. This happened because of a bad optimisation; adding a file when it's already present and unchanged was optimised to do nothing. But if the file had just been removed, and was put back, this resulted in the removal being staged, and the add not being staged. Ugly bug, although the assistant's daily sanity check automatically restaged the files.

Underlying that bug was a more important problem: git-annex does not always update working tree files atomically. So a crash at just the wrong instant could cause a file to be deleted from the working tree. I fixed that too; all changes to files in the working tree should now be staged in a temp file, which is renamed into place atomically.

Also made a bunch of improvements to the dashboard's transfer display, and to the handling of the underlying transfer queue.