Allow merging unrelated histories in git rebase

Question

When you want to rebase a branch keeping merge commits you pass the --preserve-merges flag. When you are merging unrelated history in git you need to pass the --allow-unrelated-histories flag.

If you are doing git rebase --preserve-merges when an existing merge comes from an unrelated history, it fails:

fatal: refusing to merge unrelated histories

If you try git rebase --preserve-merges --allow-unrelated-histories it fails with:

error: unknown option 'allow-unrelated-histories'

Is there some other way to tell rebase to allow the merge?

---

Edit: here is a minimal reproduction: https://github.com/vossad01/rebase-unrelated-merge-reproduction

To reproduce checkout master then execute:

git rebase --preserve-merges --onto origin/a-prime HEAD~2

Answer 1

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.

Answer 2

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