Recorded demos with a merge commit play back

A demo whose agents ran for real once can be played back in later takes. Until now the playback refused when an agent had made a merge commit, for example merging main into a pull request to resolve a conflict: its commits were replayed one by one, which flattened the merge into a conflict. The recorded commits now go into the workspace as recorded, merges included, built on the pull request and main of the current take, so such a demo replays like any other.