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;