bug#15163: 24.3; python-mode, electric colon bug

[Stefan Monnier]

> if a:
>     if b:
>         pass
>     else:
>         pass
>         else:

> The last 'else' isn't indented properly.

Fabián, could you look into it?
When tracing through python-indent-electric-colon, I see that
python-info-closing-block-message returns nil, so we indent back to the
original indentation.
And while you're looking at this code, could you add a comment
explaining why you do

      (python-info-closing-block-message)
      (when (> indentation calculated-indentation)
        (save-excursion
          (indent-line-to calculated-indentation)
          (when (not (python-info-closing-block-message))
            (indent-line-to indentation)))))))

instead of (say)
      
      (when (and (python-info-closing-block-message)
                 (> indentation calculated-indentation))
        (save-excursion
          (indent-line-to calculated-indentation)))


-- Stefan