Add support for lightweight tags

There is nothing bad about a tag that has no tag-object, but the old code
didn't handle such tags correctly. Fix it.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

git.h

@@ -278,6 +278,10 @@ struct object {
 };
 
 
+/** Returns the object, having parsed it to find out what it is. **/
+struct object *parse_object(const unsigned char *sha1);
+
+
 /*
  * from git:tree.h
  */