pull: reorganize bundle2 argument bundling We are about to add the ability to use stream bundle with bundle2. Before doing so, we need to gather some code that will not be used in the bundle2 case. There is no behavior change within this changeset.

clone: allow bundle2's stream clone with 'server.disablefullbundle' The previous check was a bit too strict and would not recognize a get bundle not requesting changegroup.

bundle2: add support for a 'stream' parameter to 'getbundle' This parameter can be used to request a stream bundle.