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- Pull changes from the ACME subtree into a new branch (git subtree split -P <external_subdir> -b <external_branchname>).
- If necessary, add remote for updated external (If git remote does not list your remote, add it with git remote add -f --tags <external_name> <external_url>).
- Similarly, add a remote for <external_repo> if different from <external_name>
- Push your branch to the external's repo, in preparation of merging following their workflow: (git push <external_repo> <external_branchname>)
- Push the branch to the remote repository (git push -u <external_repo> <external_mergebranch>).
- Follow other procedures for updating the external (depending on that external's workflow).
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