gitk: set minimum size on configuration dialog

gitk sets no size limit on its configuration dialog, allowing the user to collapse the window so almost nothing is visible. The geometry manager sets an initial size so all the widgets are visible, though ignores the potentially very long text in the entry widgets in doing so. Let's use this initial size as the minimum. The size information is computed in Tk's idle processing queue, so a wait is required. Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>