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>

gitk

@@ -11894,6 +11894,10 @@ proc doprefs {} {
     grid $top.buts.ok $top.buts.can -padx 20
     grid $top.buts -sticky w -pady 10
     bind $top <Visibility> [list focus $top.buts.ok]
+
+    # let geometry manager determine run, set minimum size
+    update idletasks
+    wm minsize $top [winfo reqwidth $top] [winfo reqheight $top]
 }
 
 proc choose_extdiff {} {