pull: use 'phase-heads' to retrieve phase information A new bundle2 capability 'phases' has been added. If 'heads' is part of the supported value for 'phases', the server supports reading and sending 'phase- heads' bundle2 part. Server is now able to process a 'phases' boolean parameter to 'getbundle'. If 'True', a 'phase-heads' bundle2 part will be included in the bundle with phase information relevant to the whole pulled set. If this method is available the phases listkey namespace will no longer be listed. Beside the more efficient encoding of the data, this new method will greatly improve the phase exchange efficiency for repositories with non-served changesets (obsolete, secret) since we'll no longer send data about the filtered heads. Add a new 'devel.legacy.exchange' config item to allow fallback to the old 'listkey in bundle2' method. Reminder: the pulled set is not just the changesets bundled by the pull. It also contains changeset selected by the "pull specification" on the client side (eg: everything for bare pull). One of the reason why the 'pulled set' is important is to make sure we can move -common- nodes to public.