patch 8.1.1880: cannot show extra info for completion in a popup window

Problem:    Cannot show extra info for completion in a popup window.
Solution:   Add the "popup" entry in 'completeopt'.
diff --git a/src/search.c b/src/search.c
index da64e65..46273e1 100644
--- a/src/search.c
+++ b/src/search.c
@@ -5578,7 +5578,7 @@
 		    if (g_do_tagpreview != 0)
 		    {
 			curwin_save = curwin;
-			prepare_tagpreview(TRUE);
+			prepare_tagpreview(TRUE, TRUE, FALSE);
 		    }
 #endif
 		    if (action == ACTION_SPLIT)