This kind of test was proposed by led@ to gather statistics
on chroot's contents going to become squashfs (the script
optimizations lowering added overhead from ~10 sec down
to a subsecond range were also proposed by him).

Intentionally not documented in doc/variables.txt due to
the rather lowlevel nature of the probe (at least so far).

The knobs involved are SQUASHFS (the additional effort kicks
in only for "tight" case) and GLOBAL_SQUASHFS_SORT (must be
non-empty for this extra overhead to occur).

Additional experimentation is needed to find out whether
the difference in squashfs size and performance is worth
the trouble (seems the impact is non-zero but pretty minor).