Improve message-id generation flow control for format-patch

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/revision.h b/revision.h
index 8572315..e3559d0 100644
--- a/revision.h
+++ b/revision.h
@@ -74,7 +74,7 @@
 	struct log_info *loginfo;
 	int		nr, total;
 	const char	*mime_boundary;
-	const char	*message_id;
+	char		*message_id;
 	const char	*ref_message_id;
 	const char	*add_signoff;
 	const char	*extra_headers;