mu: use sha-1 for fallback msgid

We were using a hash based on the path, but it's better to have
something that's constant even if the path chnages.

lib/mu-msg-priv.hh

@@ -36,6 +36,7 @@ struct MuMsgFile {
 	size_t		 _size;
 	char	         *_path;
 	char		 *_maildir;
+        char             *_sha1;
 };