fetch(find_ghosts=True) may not find all CHKMap pages

[Martin Pool]

Following on from https://bugs.edge.launchpad.net/bzr/+bug/391411 - fetching with find_ghosts=True is supposed to bring across all available data, passing down through revisions already present in the destination repository. However, when it's used to fetch data into a previously stacked repository, it may terminate too early by seeing that a revision is present, when in fact some of the CHKMap pages for that revision's inventory are not present.

If this actually occurs, it will probably be seen as being unable to reconstruct some revisions from the repository after unstacking. It may also occur in other fetch cases.