How to make next git commit to exactly represent project state from another commit without merge?

Question

I have two git branches: Alpha and Beta and I need to create next commit on the branch Alpha so its project state would be exactly the same like on branch Beta. Without merge.

I understand that every git commit - on a logical level - is a complete and immutable project state + some metadata. So I just need branch Alpha to have the same state of files like Beta does.

It would be another commit (with different hash), and one branch tip wouldn't reference another.

Answer 1

c=$(git commit-tree Beta^{tree} -p Alpha -m 'foo')
test $c && git update-ref refs/heads/Alpha $c

git commit-tree creates a new commit, whose tree is the same with Beta's tip's and whose parent is Alpha's tip.

git update-ref makes Alpha point at the new commit.

Answer 2

But it I believe there should be a simpler and more elegant way.

There is a way which does not involve patches: reset --hard followed by reset --soft.

• First, mark the current HEAD of your branch: we will need to move that HEAD without modifying Alpha, so let's create a new temporary branch called 'tmp' where Alpha is.

  git checkout Alpha
  git checkout -b tmp
  

• Then we go back to an Beta HEAD commit with the right content.

  git reset --hard Beta 
  

That resets the index (and working tree) to the right content, but that also moves HEAD. However, that moves tmp HEAD, not Alpha HEAD.

• move back tmp HEAD to where mAlphaaster is, but without modifying the index or the working tree (which are representing what we need for a new commit)

  git reset --soft Alpha
  

• make a new commit, on top of Alpha/tmp HEAD, which represents the right content (Beta HEAD commit content).

  git commit -m "new commit, image of an old one"
  

• Finally, force Alpha to be where tmp is: one new commit later.

  git branch -M tmp Alpha
  git checkout Alpha
  git branch -d tmp
  

Now a regular git push is enough, any collaborator can simply pull as usual, and still get the old reset content.

git push