patch 8.2.3921: the way xdiff is used is inefficient

Problem:    The way xdiff is used is inefficient.
Solution:   Use hunk_func instead of the out_line callback. (Lewis Russell,
            closes #9344)
diff --git a/src/version.c b/src/version.c
index 8963062..96359c1 100644
--- a/src/version.c
+++ b/src/version.c
@@ -750,6 +750,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    3921,
+/**/
     3920,
 /**/
     3919,