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: NShawn O. Pearce <spearce@spearce.org>

lib/choose_rev.tcl

@@ -451,7 +451,8 @@ method _sb_set {sb orient first last} {
 			focus $old_focus
 		}
 	}
-	$sb set $first $last
+
+	catch {$sb set $first $last}
 }
 
 method _show_tooltip {pos} {