gitk: set minimum size on configuration dialog

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>