Rebasing a branch that has been branched from

Question

I'm attempting to cut off a part of the history of a repository so I can graft it back onto the repository with git replace as described in this article .

The main difference between the case described in the article and my repository is, that my master branch has existing branches. I would like to keep the layout of those branches and their branch-points equivalent after I have cut away the history.

My history approximately looks like this

    cut here
      |
      v
o--o--o--o--o--o--o master
            \  \  \
             \  \  o--o   A
              \  o--o--o  B
               o--o--o    C

There are no branches off of master before the "cut here" commit.

What I want to achieve is this:

o--o--o history

initial tree at cut
        |
        v
        o--o--o--o--o master
               \  \  \
                \  \  o--o   A
                 \  o--o--o  B
                  o--o--o    C

That means it wouldn't be sufficient to just rebase master on the initial tree but I also need to replay all existing branch on their equivalent new commit after the rebase of master. I don't want to just replay all branches on the new master, because that would possibly have conflicts.

Is there any smart and hopefully automatic way to do this?

Answer 1

These commands will have the result you are looking for:

git checkout --orphan temp-branch CUT–COMMIT
git commit // This will create your new initial commit
git rev-parse HEAD // To get the SHA of this commit
git replace CUT-COMMIT NEW-COMMIT
git filter-branch -- --all

If you have any question regarding to how and why this works, feel tree to leave a comment. You might want to backup your repo before you do stuff like this.

From the filter-branch docu :

NOTE: This command honors .git/info/grafts and .git/refs/replace/. If you have any grafts or replacement refs defined, running this command will make them permanent.