patch 8.2.4707: redrawing could be a bit more efficient

Problem:    Redrawing could be a bit more efficient.
Solution:   Optimize redrawing. (closes #10105)
diff --git a/src/change.c b/src/change.c
index afc32b7..bc9195b 100644
--- a/src/change.c
+++ b/src/change.c
@@ -640,9 +640,13 @@
 	    if (hasAnyFolding(wp))
 		set_topline(wp, wp->w_topline);
 #endif
-	    // Relative numbering may require updating more.
+	    // If lines have been added or removed, relative numbering always
+	    // requires a redraw.
 	    if (wp->w_p_rnu && xtra != 0)
-		redraw_win_later(wp, SOME_VALID);
+	    {
+		wp->w_last_cursor_lnum_rnu = 0;
+		redraw_win_later(wp, VALID);
+	    }
 #ifdef FEAT_SYN_HL
 	    // Cursor line highlighting probably need to be updated with
 	    // "VALID" if it's below the change.