Context: The order of stacked windows is: menu, details, diff. The
stacking window width was set sequentially for each window and relied
only on the buffer content of the last window and the current window.
This meant that when the diff window content was wider than the details
window content, the diff window resized all windows in its stack (menu,
details) to match the width of the diff window.

Problem: This logic ignores the fact that the menu window could be wider
than the details window. In such a situation, when the details window
was being opened, it adjusted its width to match the width of the wider
menu window. However, since the diff window only looked at buffer
contents of the details window, and not at the actual window width of
the details window, it assumed that the diff window is the widest one,
and set the width of the menu window and the details window to match the
diff window. This makes the menu window narrower than its content.

Solution: Change the `StackingWindow:after_opened()` window resize logic
to look at the *actual* window width, rather than the buffer content
width. When `after_opened()` runs for the diff window, it correctly can
identify that the details window was resized due to the menu window
being wider, and correctly makes all windows' width be `max(diff window,
previous windows)`.

Fixes weilbith#52