Problem:
- If cmdwin window is split, ENTER in one does not close the others.
- If cmdwin is put into a different tabpage via <c-w>T, it stops working
(ENTER does not execute the cmd).
Solution:
- Close the buffer instead of the window.
- In the WinClosed handler, skip `M._cleanup()` unless this is the last
cmdwin window.
684371ba5d
