patch 8.0.0205: wrong behavior after :undojoin

Problem:    After :undojoin some commands don't work properly, such as :redo.
            (Matthew Malcomson)
Solution:   Don't set curbuf->b_u_curhead. (closes #1390)
diff --git a/src/undo.c b/src/undo.c
index 607f35f..b69f318 100644
--- a/src/undo.c
+++ b/src/undo.c
@@ -3136,11 +3136,8 @@
     if (get_undolevel() < 0)
 	return;		    /* no entries, nothing to do */
     else
-    {
-	/* Go back to the last entry */
-	curbuf->b_u_curhead = curbuf->b_u_newhead;
-	curbuf->b_u_synced = FALSE;  /* no entries, nothing to do */
-    }
+	/* Append next change to the last entry */
+	curbuf->b_u_synced = FALSE;
 }
 
 /*