vim-patch:9.1.0049: Make "[Command Line]" a special buffer name

Problem:  E95 is possible if a buffer called "[Command Line]" already
          exists when opening the cmdwin. This can also happen if the
          cmdwin's buffer could not be deleted when closing.

Solution: Un-name the cmdwin buffer, and give it a special name instead,
          similar to what's done for quickfix buffers and for unnamed
          prompt and scratch buffers. As a result, BufFilePre/Post are
          no longer fired when opening the cmdwin. Add a "command" key
          to the dictionary returned by getbufinfo() to differentiate
          the cmdwin buffer instead. (Sean Dewar)

Cherry-pick test_normal changes from v9.0.0954.

1fb4103206

src/nvim/eval/buffer.c

@@ -494,6 +494,7 @@ static dict_T *get_buffer_info(buf_T *buf)
   tv_dict_add_nr(dict, S_LEN("changed"), bufIsChanged(buf));
   tv_dict_add_nr(dict, S_LEN("changedtick"), buf_get_changedtick(buf));
   tv_dict_add_nr(dict, S_LEN("hidden"), buf->b_ml.ml_mfp != NULL && buf->b_nwindows == 0);
+  tv_dict_add_nr(dict, S_LEN("command"), buf == cmdwin_buf);
 
   // Get a reference to buffer variables
   tv_dict_add_dict(dict, S_LEN("variables"), buf->b_vars);