bundle2: add an exchange.getbundle function This function can return a `HG10` or `HG20` bundle. It uses the `bundlecaps` parameters to decides which one to return. This is a distinct function from `changegroup.getbundle` for two reasons. First the APIs of `bundle10` and `bundle20` are not compatible yet. The two functions may be reunited in the future. Second `exchange.getbundle` will grow parameters for all kinds of data (phases, obsmarkers, ...) so it's better to keep the changegroup generation in its own function for now. This function will be used it in the next changesets.