Occasional AttributeError from Tree.get_file_by_path

Tree.get_file_by_path tries to use self._inventory.path2id instead of
self.path2id which may cause an AttributeError exception to be raised when
self._inventory is None. E.g. WorkingTree4+ objects reimplement self.path2id
so it doesn't use self._inventory.

I think Tree.get_file_by_path would use self.path2id which should make the
method work for WorkingTree4+ objects as well.

I'm not certain in which exact situations this is a problem, I encountered it
while attempting some modifications in bzr-upload plugin.

See ml for more info and discussion:
https://lists.ubuntu.com/archives/bazaar/2010q4/070748.html