bundle2: detect bundle2 stream/request on /HG2./ instead of /HG2Y/ To support more bundle2 formats, we need a wider detection of bundle2-family streams. The various places what were explicitly detecting the full magic string are now matching on the first three characters of it.

unbundle20: allow generic dispatch between unbundlers We now take full advantage of the 'getunbundler' function by using a '{version -> unbundler-class}' mapping. This map currently contains a single entry but will make it easy to support more versions from an extension/the future. At some point, this map will probably contain bundler-class information too, in the same fashion the packer map does. However, this is not critically required right now so it will happen by itself when needed. The main target is to allow HG2Y support in an extension to ease transition of companies using the experimental protocol in production (yeah...) But I've no doubt this will be useful when playing with a future HG21.

treemanifest: refactor treemanifest.walk() This refactor is a preparation for an optimization in the next commit. This introduces a recursive element that recurses each submanifest. By using a recursive function, the next commit can avoid walking over some subdirectories altogether.