[MERGE] Add a simple status of two historical trees.

[Andrew Bennetts]

Robert Collins wrote:
> This adds a simple 'st -r -2..-1' call to smoke-test performance of
> historical trees directly. log -v testing should show this too, but I
> think this is valuable to have because it does much less work.

Sounds good to have.  The patch for it looks sane to me too.

> once I understand a bit more of usertest, doing st -r -200..-1 would be
> good, but I don't know if that much history is reliably available...

usertest is seems designed to work with user-supplied datasets, so that much
history isn't necessarily going to be available.  Perhaps the simplest way to
deal with this is make a new script/suite for deep history tests, and then just
rely on the user to only run that suite with an appropriate data set?

-Andrew.