Allow merging unrelated histories in git rebase

When git rebase fails on the merge it does not abort the rebase, so you have the opportunity to manually intervene.

If you are willing to to resolve this by hand, you can complete the merge as follows:

git merge --allow-unrelated ORIGINAL_BRANCH_THAT_WAS_MERGED --no-commit
git commit -C ORIGINAL_MERGE_COMMIT
git rebase --continue

Ideally, there would be a way for Git to handle this without manual intervention.

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