rev-list --objects: fix object list without commit.

Earlier, "rev-list --objects <sha1>" for an object chain that
does not have any commit failed with a usage message.  This
fixes "send-pack remote $tag" where tag points at a non-commit
(e.g. a blob).

Signed-off-by: Junio C Hamano <junkio@cox.net>

rev-list.c

@@ -850,7 +850,8 @@ int main(int argc, const char **argv)
 		handle_one_commit(commit, &list);
 	}
 
-	if (!list)
+	if (!list &&
+	    (!(tag_objects||tree_objects||blob_objects) && !pending_objects))
 		usage(rev_list_usage);
 
 	paths = get_pathspec(prefix, argv + i);