git-gui: Work around random missing scrollbar in revision list

If the horizontal scrollbar isn't currently visible (because it has not been needed) but we get an update to the scroll port we may find the scrollbar window exists but the Tcl command doesn't. Apparently it is possible for Tk to have partially destroyed the scrollbar by removing the Tcl procedure name but still leaving the widget name in the window registry. Signed-off-by: Shawn O. Pearce <spearce@spearce.org>