vim-patch:8.2.0614: get ml_get error when deleting a line in 'completefunc' (#19244)

Problem:    Get ml_get error when deleting a line in 'completefunc'. (Yegappan
            Lakshmanan)
Solution:   Lock the text while evaluating 'completefunc'.
ff06f283e3

Fix a mistake in the porting of patch 8.1.0098.
Cherry-pick Test_run_excmd_with_text_locked() from patch 8.2.0270.
Cherry-pick test_gf.vim changes from patch 8.2.0369.
Cherry-pick message change from later patches.

src/nvim/undo.c

@@ -297,7 +297,7 @@ bool undo_allowed(buf_T *buf)
   // Don't allow changes in the buffer while editing the cmdline.  The
   // caller of getcmdline() may get confused.
   if (textlock != 0) {
-    emsg(_(e_secure));
+    emsg(_(e_textlock));
     return false;
   }