Unknown Option 'Allow-Unrelated-Histories'

unknown option `allow-unrelated-histories'

I documented before how that option has been introduced in Git 2.9, June 2016 (as mentioned by merlin2011 in the comments)

Since Ubuntu LTS 14.04 comes with an old 1.9+ Git, you need to reference an up-to-date ppa:

sudo add-apt-repository ppa:git-core/ppa
sudo apt-get update
sudo apt install git

That ppa (Personnal Archive Package) is the git-core/+archive/ubuntu/ppa, and will include the latest Git 2.11 release.

Allow merging unrelated histories in git rebase

The brute-force method is to force a common root -- since you're trying to rebase roots, with no content history, make a nonce empty commit and tell git that's the parent of the histories you're merging:

git rev-list --all --max-parents=0 \
| awk '{print $0,empty}' empty=`:|git mktree|xargs git commit-tree` \
> .git/info/grafts
git rebase here
rm .git/info/grafts

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