changegroup: introduce cg3, which has support for exchanging treemanifests I'm not entirely happy with using a trailing / on a "file" entry for transferring a treemanifest. We've discussed putting some flags on each file header[0], but I'm unconvinced that's actually any better: if we were going to add another feature to the cg format we'd still be doing a version bump anyway to cg4, so I'm inclined to not spend time coming up with a more sophisticated format until we actually know what the next feature we want to stuff in a changegroup will be. Test changes outside test-treemanifest.t are only due to the new CG3 bundlecap showing up in the wire protocol. Many thanks to adgar@google.com and martinvonz@google.com for helping me with various odd corners of the changegroup and treemanifest API. 0: It's not hard refactoring, nor is it a lot of work. I'm just disinclined to do speculative work when it's not clear what the customer would actually be.