Dreadfully early this morning I developed a plan for a way to finish the
last v6 blocker, that works around most of the problems with git's smudge 
interface. The only problem with the plan is that it would make both `git
stash` and `git reset --hard` leave unlocked annexed files in an
unpopulated state when their content is available. The user would have to
run git-annex afterwards to fix up after them. All other git checkout,
merge, etc commands would work though.

Not sure how I feel about this plan, but it seems to be the best one so
far, other than going off and trying to improve git's smudge interface again.
I also wrote up [[git_smudge_clean_interface_suboptiomal]] which explains
the problems with git's interface in detail.