patch 8.1.1789: cannot see file name of preview popup window

Problem:    Cannot see file name of preview popup window.
Solution:   Add the file name as the title.
diff --git a/src/ex_cmds.c b/src/ex_cmds.c
index 57c98ae..f79f054 100644
--- a/src/ex_cmds.c
+++ b/src/ex_cmds.c
@@ -3172,6 +3172,10 @@
 #ifdef FEAT_TITLE
 	maketitle();
 #endif
+#ifdef FEAT_TEXT_PROP
+	if (popup_is_popup(curwin) && curwin->w_p_pvw)
+	    popup_set_title(curwin);
+#endif
     }
 
 #ifdef FEAT_DIFF