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)

src/version.c

@@ -749,6 +749,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3921,
 /**/
     3920,
 /**/