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/testdir/test_popupwin.vim b/src/testdir/test_popupwin.vim
index 1a39d28..7b7b1bd 100644
--- a/src/testdir/test_popupwin.vim
+++ b/src/testdir/test_popupwin.vim
@@ -525,6 +525,9 @@
 
   call term_sendkeys(buf, ":call popup_close(winid)\<CR>")
   call term_sendkeys(buf, "\"*p")
+  " clean the command line, sometimes it still shows a command
+  call term_sendkeys(buf, ":\<esc>")
+
   call VerifyScreenDump(buf, 'Test_popupwin_select_02', {})
 
   " clean up
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,