refs: convert dwim_ref and expand_ref to struct object_id

All of the callers of these functions just pass the hash member of a
struct object_id, so convert them to use a pointer to struct object_id
directly.  Insert a check for NULL in expand_ref on a temporary basis;
this check can be removed when resolve_ref_unsafe is converted as well.

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

archive.c

@@ -371,7 +371,7 @@ static void parse_treeish_arg(const char **argv,
 		const char *colon = strchrnul(name, ':');
 		int refnamelen = colon - name;
 
-		if (!dwim_ref(name, refnamelen, oid.hash, &ref))
+		if (!dwim_ref(name, refnamelen, &oid, &ref))
 			die("no such ref: %.*s", refnamelen, name);
 		free(ref);
 	}