Not a lot of progress on [[todo/import_tree]] today I feel..

Started off by adding a QuickCheck test of the content
identifier log, which did find one bug in that code.

Then started roughing out the core of the importing operation, which involves
building up git trees for the files that are imported. But that needs a
way to graft an imported tree into a subdirectory of another tree, 
and the only way I had available to do it needed to read in the entire
recursive tree of the current branch, which would be slower and use
more memory than I like.

So, got sidetracked building a git tree grafter. It turns out that 
the export tree code also needs to graft a tree (into the git-annex
branch), and did so using the same innefficient method that I want to
avoid, so it will also be able to be improved using the grafter.

Unfortunately, I had to stop for the day with the grafter not quite working
properly.