patch 9.1.0948: Missing cmdline completion for :pbuffer

Problem:  Missing cmdline completion for :pbuffer.
Solution: Add cmdline completion for :pbuffer like :buffer.
          (zeertzjq)

fixes: #16250
closes: #16251

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_preview.vim b/src/testdir/test_preview.vim
index 0df1a90..7c7883a 100644
--- a/src/testdir/test_preview.vim
+++ b/src/testdir/test_preview.vim
@@ -47,12 +47,23 @@
   edit Xpreview.c
   const buf_num = bufnr('%')
   enew
+
+  call feedkeys(":pbuffer Xpre\<C-A>\<C-B>\"\<CR>", 'xt')
+  call assert_equal("\"pbuffer Xpreview.c", @:)
+
   call assert_equal(1, winnr('$'))
   exe 'pbuffer ' .  buf_num
   call assert_equal(2, winnr('$'))
   call assert_equal(0, &previewwindow)
 
   call s:goto_preview_and_close()
+
+  call assert_equal(1, winnr('$'))
+  pbuffer Xpreview.c
+  call assert_equal(2, winnr('$'))
+  call assert_equal(0, &previewwindow)
+
+  call s:goto_preview_and_close()
 endfunc
 
 func Test_window_preview_terminal()