Remove a commit's parent, thereby making it the initial commit

Question

Note: I know this is rewriting history, will change all the hashes, and mess everyone else up.

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I want to take a commit, and remove its parent. Specifically, the commit should now look like an initial commit. That means its diff will change; it will look as though all the files were created by that commit, at that point in history.

How does one do this? (I could, of course, go edit the commit object, but then the commits wouldn't point to each other.) Also, I would like to do this on a new branch (meaning that there are now two histories: the original one, and another where the commit is the initial commit.)

Answer 1

You can't modify an existing commit without changing its hash. But you can make a new commit that looks like the old commit, but without its parents.

Here, I am creating a new commit with a new commit message, but copying the tree of the current HEAD:

$ git commit-tree -m "New commit message" "HEAD^{tree}"
fe7b5f20e7c42550d81650ee75efb654cdbfeb11

This will print out the commit ID of the new commit.

If you want to make the current branch (e.g: master) point to this new commit with new parents, you can do it all in one step like this:

$ git reset --hard $(git commit-tree -m "New commit message" "HEAD^{tree}")

Answer 2

git rebase -i HEAD~

Then in the editor that pops up after the above terminal cmd Change the start of child commit line from pick to squash. Or 'p' and 's' respectively. Write the file and exit the editor that appeared upon the above command

Edit: Assuming the two commits are the most recent. If not then you can check out the child's child commit hash and use git rebase -i HEAD~2 instead

Edit2: or you could just git rebase -i parentcommithash and the top of the list should be the parent

Answer 3

Let's say you want your new initial commit to be 123456.

git checkout -b new_master
git filter-branch --commit-filter '
  if git merge-base --is-ancestor 123456 $GIT_COMMIT ;
  then
   git commit-tree "$@";
  else
   skip_commit "$@";
  fi' HEAD

Todo (for anyone): Edit in an explanation

Answer 4

git checkout -b newbranch
git rev-parse @ >.git/info/grafts   # any equivalent for @ will work e.g. HEAD or newbranch
git filter-branch
rm .git/info/grafts

(edit: added the rm. Without that, this repo will still reflect the grafted ancestry for the original commit)

Answer 5

It seems what you want is to squash all commits since your initial commit up until (and including) the commit you want to be the new initial commit.

You can do this with git rebase --interactive --root. This will give you a todo file with all commits. Then go on and edit all the commits from the second to squash, it would look something like this:

pick <hash> initial commit
squash <hash> want change but not commit
squash <hash> want change but not commit
squash <hash> want change but not commit
squash <hash> want change but not commit
squash <hash> this will be new init
pick <hash> this will be the second commit
pick <hash> and so on...

You will be prompted to write the commit message for the resulting squash.