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>

src/cmdexpand.c

@@ -2332,6 +2332,7 @@ set_context_by_cmdname(
 	    // FALLTHROUGH
 	case CMD_buffer:
 	case CMD_sbuffer:
+	case CMD_pbuffer:
 	case CMD_checktime:
 	    xp->xp_context = EXPAND_BUFFERS;
 	    xp->xp_pattern = arg;

src/testdir/test_preview.vim

@@ -47,12 +47,23 @@ func Test_window_preview_from_pbuffer()
   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()

src/version.c

@@ -704,6 +704,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    948,
 /**/
     947,
 /**/