patch 8.1.1668: popup window test is a bit flaky on some systems

Problem:    Popup window test is a bit flaky on some systems.
Solution:   Clear the command line. (Naruhiko Nishino, closes #4656)
diff --git a/src/version.c b/src/version.c
index afd775a..a2137cc 100644
--- a/src/version.c
+++ b/src/version.c
@@ -778,6 +778,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1668,
+/**/
     1667,
 /**/
     1666,