'bzr reconcile' unsafe for stacked 2a? (ignores parent inventories)

Compare GCCHKPacker and GCCHKReconcilePacker:

    def _copy_inventory_texts(self):
        source_vf, target_vf = self._build_vfs('inventory', True, True)
        # It is not sufficient to just use self.revision_keys, as stacked
        # repositories can have more inventories than they have revisions.
        # One alternative would be to do something with
        # get_parent_map(self.revision_keys), but that shouldn't be any faster
        # than this.
        inventory_keys = source_vf.keys()
        missing_inventories = set(self.revision_keys).difference(inventory_keys)
        if missing_inventories:
            missing_inventories = sorted(missing_inventories)
            raise ValueError('We are missing inventories for revisions: %s'
                % (missing_inventories,))
        self._copy_stream(source_vf, target_vf, inventory_keys,
                          'inventories', self._get_filtered_inv_stream, 2)

vs.

    def _copy_inventory_texts(self):
        source_vf, target_vf = self._build_vfs('inventory', True, True)
        self._copy_stream(source_vf, target_vf, self.revision_keys,
                          'inventories', self._get_filtered_inv_stream, 2)
        if source_vf.keys() != self.revision_keys:
            self._data_changed = True

Off the top of my head I'd expect the missing_inventories logic is just as necessary in the reconcile case, so at a glance it looks like 'bzr reconcile' on a stacked repository could discard parent inventories, leading to the old 'AbsentContentFactory' bug fetching from affected repositories.

(If my understanding is wrong then I'd say there's still a bug: that the code needs a comment that explains why it is reasonable for these two implementations of _copy_inventory_texts to diverge so much.)

I'll propose a branch making what I think is the obvious fix for discussion. I haven't written a test yet because the scenario seems pretty fiddly to create, but perhaps it's easier than it looks.