patch 8.2.0614: get ml_get error when deleting a line in 'completefunc'

Problem:    Get ml_get error when deleting a line in 'completefunc'. (Yegappan
            Lakshmanan)
Solution:   Lock the text while evaluating 'completefunc'.
diff --git a/src/undo.c b/src/undo.c
index c5ce306..c11b048 100644
--- a/src/undo.c
+++ b/src/undo.c
@@ -333,7 +333,7 @@
     // caller of getcmdline() may get confused.
     if (textlock != 0)
     {
-	emsg(_(e_secure));
+	emsg(_(e_textlock));
 	return FALSE;
     }