Test the current state of the cache-tree optimization

Test the current state of the cache-tree optimization

The cache-tree optimization originally helped speed up write-tree
operation.  However, many commands no longer properly maintain -- or
use an opportunity to cheaply generate -- the cache-tree data.  In
particular, this affects commit, checkout and reset.  The notable
examples that *do* write cache-tree data are read-tree and write-tree.

This sadly means most people no longer benefit from the optimization,
as they would not normally use the plumbing commands.

Document the current state of affairs in a test file, in preparation
for improvements in the area.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>