bundlerepo: add support for treemanifests in cg3 bundles This is a little messier than I'd like, and I'll probably come back and do some more refactoring later, but as it is this unblocks narrowhg. An alternative approach (which I may do as part of the mentioned refactoring) would be to construct *all* dirlog instances up front, so that we don't have to keep track of the linkmapper method. This would avoid a reference cycle between the bundlemanifest and the bundlerepository, but I was hesitant to do all the work up front like that. With this change, it's possible to do 'hg incoming' and 'hg pull' from bundles in .hg/strip-backup in a treemanifest repository. Sadly, this doesn't make it possible to 'hg clone' one of those (if you do 'hg strip 0'), because the cg3 in the bundle gets written without a treemanifest flag. Since that's going to be an involved refactor in a different part of the code (which I *suspect* won't touch any of the code I've just written here), let's leave it as an idea for Later.