Incorrect margins in log scale #6643

As of 2.0b1 and master, semilogy([.2, 2]) gives

i.e. the bottom margin is too small.

Actually, handle_single_margin (in _AxesBase) does the right thing (expand the limits after transforming to plot coordinates), but then passes the limits to LogLocator.view_limits, which undoes all the good work while executing

        if vmin <= minpos:
            vmin = minpos

(this check was introduced by 426eabc, let's see whether it's still needed...)