shortlog: extract --group fragment for translation

The subsequent commit will add another unhandled case in
`read_from_stdin()` which will want to use the same message as with
`--group=trailer`.

Extract the "--group=trailer" part from this message so the same
translation key can be used for both cases.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/shortlog.c

@@ -132,7 +132,7 @@ static void read_from_stdin(struct shortlog *log)
 		match = committer_match;
 		break;
 	case SHORTLOG_GROUP_TRAILER:
-		die(_("using --group=trailer with stdin is not supported"));
+		die(_("using %s with stdin is not supported"), "--group=trailer");
 	default:
 		BUG("unhandled shortlog group");
 	}