This will help keep the ElectrumABC github repo up to date, which is needed because we need to push tags there to host the releases. It is also convenient for troubleshooting and git-blaming to have a repository with an uninterrupted git history.
Note that the process does not remove the files that were removed in the `[electrum] initial monorepo integration` commit (cirrus.yml, pre-commit-config.yaml...). This will have to be done by manually pushing a commit to the github repo, but I don't anticipate it causing any future conflict.