patch 8.1.0709: windows are updated for every added/deleted sign

Problem:    Windows are updated for every added/deleted sign.
Solution:   Do not call update_debug_sign().  Only redraw when the line with
            the sign is visible.  (idea from neovim #9479)
diff --git a/src/version.c b/src/version.c
index 3375b1c..e1ea65e 100644
--- a/src/version.c
+++ b/src/version.c
@@ -800,6 +800,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    709,
+/**/
     708,
 /**/
     707,