should have tests that bisect only part of a history

[Samuel Bronson]

All of the full bisection runs currently in bzr-bisect's test suite appear to bisect the full history of the test branch; that is, they all start with something like this:

    bzr bisect start
    bzr bisect no
    bzr bisect yes -r 1

and, since the working tree begins at the most recent revision, this means bisecting the whole branch. (All five mainline commits of it!)

This seems likely to miss some potential problems...