Directory deltification

As of r1224839, FSFS now supports directory deltification.
Please review the changes and run tests against different
repositories so that we get a better idea of what the costs
and benefits are. As soon as I'm back home, I will run tests
against the Apache and KDE repositories.

So far, I ran tests against the rather small TSVN repository.
It seems that we get 50% more capacity / 33% size savings
for 0 .. 20% CPU overhead. The savings should be more
significant on larger repositories and some of the extra
overhead should be removed by the file handle caching
code - once merged into /trunk.

In any case, we are still much faster than 1.6.
Detailed results can be found in the attached document.
"/trunk" is at r1224828 (i.e. without deltification tuning.

-- Stefan^2.