export: introduce a generic way to add patch header on export Extensions currently have no easy way to add data to exported patch. This is now fixed using a generic mechanism in the same fashion used by bundle2. Tests are coming in the next changeset with its first user.

incoming: request a bundle2 when possible (BC) Incoming was using bundle1 in all cases, as bundle1 is restricted to changegroup1 and does not support general delta, this can lead to significant CPU overhead if the server is using general delta storage. We now properly request and store a bundle2 to disk. If the server include any output or error in the bundle, they will be stored on disk and replayed when the bundle is read. As 'hg incoming' is going to read the bundle right away, we call that 'good' enough and go back to the bigger plan of having general delta on by default. This was tracked as 4864

bundlerepo: indent some code to prepare next patch We are about to add a new condition. Code is indented in a separated patch for readability.