Continued working on the enhancaed smudge/clean interface in git,
incorporating feedback from the git developers.

In a spare half an hour, I made an `improved-smudge-filters` branch
that teaches git-annex smudge to use the new interface. 

Doing a quick benchmark, `git checkout` of a deleted 1 gb file took:

* 19 seconds before
* 11 seconds with the new interface
* 0.1 seconds with the new interface and annex.thin set  
  (while also saving 1 gb of disk space!)

So, this new interface is very much worthwhile.