Replace a branch with another one in a Git repository

Last week a workmate did me a pull request where he wanted to merge a fully new “develop” branch into “master”. The thing was that he realized that he has to change everything ‘cause he wanted to work with the latest version of Middleman. He has already started some work with an older version but his latest work needed the latest one.

The wrong path

The first thing I tried was to dumbly merge “develop” branch into “master” but as you may have guess if you read this, it didn’t worked. Branches were so much different that Git told me:

fatal: refusing to merge unrelated histories

That makes sense since we try to merge two branches with totally unrelated changes. Our “develop” branch was not even based on “master”, it was just a fully new branch with no ancestor.

So after a few digging on the web I tried to tell to Git that I wanted to keep changes from develop without bothering about master but merge into it:

$ git merge -s ours --no-commit master

No luck with that, it was a misunderstanding of what this command does so Git tells me:

fatal: refusing to merge unrelated histories

Ok so what? The first thing you can be tempted to do is to create a new branch based on the one you want to merge, delete “master” then rename this branch to “master”. That’s kind of hacky to me. Let’s find a real good solution to this problem.

The good path

Git is so powerful that there must be a clean way to do this. And yes there’s one.

Go to the branch you want to become the new “master”, then merge “master” by telling you don’t care about unrelated histories and what you want to keep is the work in the current branch:

$ git checkout develop
$ git merge -s ours --allow-unrelated-histories master

Then you’re good to go, you can checkout “master”, merge the new version and push it:

$ git checkout master
$ git merge develop
$ git push

That was pretty tough to figure it out so I thought writing about it would help someone and that it would help the future me when I’ll have forget about it.