patch 8.1.1922: in diff mode global operations can be very slow

Problem:    In diff mode global operations can be very slow.
Solution:   Do not call diff_redraw() many times, call it once when redrawing.
            And also don't update folds multiple times.

src/screen.c

@@ -567,6 +567,12 @@ update_screen(int type_arg)
     }
 #endif
 
+#ifdef FEAT_DIFF
+    // May have postponed updating diffs.
+    if (need_diff_redraw)
+	diff_redraw(TRUE);
+#endif
+
     if (must_redraw)
     {
 	if (type < must_redraw)	    /* use maximal type */