parsers: ensure revlog index node tree is initialized before insertion Currently, the revlog index C implementation assumes its node tree will be initialized before a new element is inserted by revnum. For example, revlog.py executes 'self.index.insert(-1, e)' in _addrevision(). This is only safe because the node tree has been initialized by a "node in self.nodemap" check made in addrevision(). (For context, this was discovered while developing an experimental revlog mixin which stores "elided nodes" via a separate code path from _addrevision(); that new code path segfaults without this patch.)