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.
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