patch 8.2.0064: diffmode completion doesn't use per-window setting

Problem:    Diffmode completion doesn't use per-window setting.
Solution:   Check if a window is in diff mode. (Dominique Pell, closes #5419)
diff --git a/src/version.c b/src/version.c
index 79f9f7c..99d59df 100644
--- a/src/version.c
+++ b/src/version.c
@@ -743,6 +743,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    64,
+/**/
     63,
 /**/
     62,