Code review comment for lp:~jml/testtools/basic-mismatch

So the whole point of mismatch is to be lazy, to allow calculating the
difference once and showing it later; i agree that much of the time
this is simple and won't be deferred; keeping the ability to do so is
extremely important, and we shouldn't make it hard for folk that do
need custom mismatch classes.

That said, sure, I've no problem with the base class being more helpful.

Deleting stuff will be tricky, we've some users that have used some of
our mismatchers directly.

-Rob