push: obsolete rather than rebase & strip when enabled This requires a few changes to wrappers.push() to use obsolescence rather than strip and to make the rebase -- which is non-destructive with obsolete active -- to no longer keep the originals. Possible future work involves no longer relying on rebase for non-outgoing revisions, and simply leaving them in the troubled state. We test this feature by adding setting obsolete_mode_tests to True in classes that push changes.

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