vim-patch:9.1.0047: issues with temp curwin/buf while cmdwin is open

Problem:  Things that temporarily change/restore curwin/buf (e.g:
          win_execute, some autocmds) may break assumptions that
          curwin/buf is the cmdwin when "cmdwin_type != 0", causing
          issues.

Solution: Expose the cmdwin's real win/buf and check that instead. Also
          try to ensure these variables are NULL if "cmdwin_type == 0",
          allowing them to be used directly in most cases without
          checking cmdwin_type. (Sean Dewar)

Reset and save `cmdwin_old_curwin` in a similar fashion.
Apply suitable changes for API functions and add Lua tests.

988f74311c

src/nvim/api/win_config.c

@@ -181,7 +181,7 @@ Window nvim_open_win(Buffer buffer, Boolean enter, Dict(float_config) *config, E
   if (!buf) {
     return 0;
   }
-  if (cmdwin_type != 0 && (enter || buf == curbuf)) {
+  if ((cmdwin_type != 0 && enter) || buf == cmdwin_buf) {
     api_set_error(err, kErrorTypeException, "%s", e_cmdwin);
     return 0;
   }