builtin/fmt-merge-msg: convert remaining code to object_id

We were using the util pointer, which is a pointer to void, as an
unsigned char pointer.  The pointer actually points to a struct
origin_data, which has a struct object_id as its first member, which in
turn has an unsigned char array as its first member, so this was valid.
Since we want to convert this to struct object_id, simply change the
pointer we're using.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fmt-merge-msg.c

@@ -485,10 +485,10 @@ static void fmt_merge_msg_sigs(struct strbuf *out)
 	struct strbuf tagbuf = STRBUF_INIT;
 
 	for (i = 0; i < origins.nr; i++) {
-		unsigned char *sha1 = origins.items[i].util;
+		struct object_id *oid = origins.items[i].util;
 		enum object_type type;
 		unsigned long size, len;
-		char *buf = read_sha1_file(sha1, &type, &size);
+		char *buf = read_sha1_file(oid->hash, &type, &size);
 		struct strbuf sig = STRBUF_INIT;
 
 		if (!buf || type != OBJ_TAG)