bundle2: add a devel option controling bundle version used for exchange We need an official way to force bundle1 to be used in test. We introduce a new option 'devel.legacy.exchange' to control this. When specified, this option will control the list of bundle version Mercurial consider when exchanging with a peer. Current valid value are 'bundle1' and 'bundle2'. Using this option in all tests will allow us to remove the 'experimental.bundle2-exp' option. We will simplify the code once the experimental option is dropped.