changegroup: allow to force delta to be against p1 This new developer option is useful to general more "generic" bundle. Without this option, a bundle generated from the repository use deltas similar to the one stored in the specific repository it was generated from. This makes performance testing a bit tricky. Using deltas similar to the final result means all delta stored in the bundle can be applied to the target repository without any further processing (except for the rare case of a full snapshot). The application of such bundles (almost) never exercises the (slower) path of searching for a new valid delta. This result in unrealistic and too favorable timing and profile. Instead, we introduce an option to make sure all revisions are stored as a delta against p1. It might not be the best generation option, but it guarantees that the content will be "generic", not favoring a specific target.