Only had a couple hours today, which were spent doing some profiling of
git-annex in situations where it has to look through a large working tree in
order to find files to act on. The top five hot spots this found are
responsible for between 50% and 80% of git-annex's total CPU use in these
situations.

The first optimisation sped up `git annex find` by around 18%.
More tomorrow..
