git-gui: with themed widgets not all window geometry is restored

Hallo,

since c80d7be we use ttk widgets for git gui. With them the position of
the Widget sizes of Unstaged Changes and Stages Changes are not restored
correctly anymore.

If you open git gui, resize the widgets and reopen the gui it will not
restore the previous size. When setting

  git config gui.usettk false

the sizes are kept. I do not know the ttk widgets that well so maybe Pat
can spot the problem quite easily. Otherwise I can have a stab at it but
it will probably take me more time.

cheers Heiko