Problem: hard to view an existing buffer in the preview window
Solution: add the :pbuffer command (Yinzuo Jiang)
Similar as `:pedit` and `:buffer` command. `:pbuffer` edits buffer [N]
from the buffer list in the preview window.
`:pbuffer` can also open special buffer, for example terminal buffer.
closes: vim/vim#16222a2a2fe841e
Cherry-pick Test_popup_and_previewwindow_dump() changes from patch
9.0.0625.
Cherry-pick Run_noroom_for_newwindow_test() changes from patches
8.2.0432 and 9.0.0363.
Co-authored-by: Yinzuo Jiang <jiangyinzuo@foxmail.com>
b03e790cdd
