patch 9.1.0934: hard to view an existing buffer in the preview window

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: #16222

Signed-off-by: Yinzuo Jiang <jiangyinzuo@foxmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_statusline.vim b/src/testdir/test_statusline.vim
index 22fd78c..73c3bee 100644
--- a/src/testdir/test_statusline.vim
+++ b/src/testdir/test_statusline.vim
@@ -220,6 +220,10 @@
   wincmd j
   call assert_match('^\[Preview\],PRV\s*$', s:get_statusline())
   pclose
+  pbuffer
+  wincmd j
+  call assert_match('^\[Preview\],PRV\s*$', s:get_statusline())
+  pclose
 
   " %y: Type of file in the buffer, e.g., "[vim]". See 'filetype'.
   " %Y: Type of file in the buffer, e.g., ",VIM". See 'filetype'.