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_vim9_cmd.vim b/src/testdir/test_vim9_cmd.vim
index 51ae7e6..900bf8a 100644
--- a/src/testdir/test_vim9_cmd.vim
+++ b/src/testdir/test_vim9_cmd.vim
@@ -2044,16 +2044,27 @@
   v9.CheckScriptFailureList(lines, ["E1356:", "E1405:"])
 enddef
 
+def s:check_previewpopup(expected_title: string)
+  var id = popup_findpreview()
+  assert_notequal(id, 0)
+  assert_match(expected_title, popup_getoptions(id).title)
+  popup_clear()
+  bw Xppfile
+  set previewpopup&
+enddef
+
 " Test for the 'previewpopup' option
 def Test_previewpopup()
   set previewpopup=height:10,width:60
   pedit Xppfile
-  var id = popup_findpreview()
-  assert_notequal(id, 0)
-  assert_match('Xppfile', popup_getoptions(id).title)
-  popup_clear()
-  bw Xppfile
-  set previewpopup&
+  s:check_previewpopup('Xppfile')
+enddef
+
+def Test_previewpopup_pbuffer()
+  set previewpopup=height:10,width:60
+  edit Xppfile
+  pbuffer
+  s:check_previewpopup('')
 enddef
 
 def Test_syntax_enable_clear()