[1/1] feat(submit:github): implement Github forge

[arxanas]

Stack:

• https://github.com/arxanas/git-branchless/pull/1184

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feat(submit:github): implement Github forge

• It's not very good. It's probably buggy since there's a huge amount of state to keep in sync between commits, branches, and pull requests.
• It's not very useful. When using Github's "Rebase and merge" button, it always rewrites the commit to update the committer signature. Then, for some reason, descendant commits can't be rebased automatically, and you have to manually rebase them.