vim-patch:9.1.0948: Missing cmdline completion for :pbuffer (#31645)

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

fixes: vim/vim#16250
closes: vim/vim#16251

3baf19a2b1

src/nvim/cmdexpand.c

@@ -2001,6 +2001,7 @@ static const char *set_context_by_cmdname(const char *cmd, cmdidx_T cmdidx, expa
     FALLTHROUGH;
   case CMD_buffer:
   case CMD_sbuffer:
+  case CMD_pbuffer:
   case CMD_checktime:
     xp->xp_context = EXPAND_BUFFERS;
     xp->xp_pattern = (char *)arg;