git - setting a commit's parent without rebase

Question

I used git-svn to create a git mirror of an SVN repository. The structure inside the SVN was a little off-standard, so git created a branch that has no common commit with the master branch.

      A---B---C topic

D---E---F---G master

I know that commit A is based off commit E and I'm pretty positive that I've fixed the issues causing git not to recognize that fact (using filter-branch). What I want to do is re-attach topic to the master branch, setting E as the parent of A:

      A---B---C topic
     /
D---E---F---G master

git-rebase doesn't seem to work for me because the diff for commit A lists the creation of a whole lot of files that already exist in master, resulting in a huge number of conflicts.
From my understanding of git just setting E as the parent of A should be enough to solve all problems.
Is this possible? If it is, how can I do it?

Answer 1

Have a look at grafts (the graft file can be found in .git/info/grafts). The format is pretty simple:

<commit sha1> <parent1 sha1> <parent2 sha1> … <parentN sha1>

This makes git believe that a commit has different parents than it actually has. Use filter-branch to make grafts permanent (so the grafts file can be removed):

git filter-branch --tag-name-filter cat -- --all

Note that this rewrites history of the repository, so should not be used on shared repos!

---

If you only want to rewrite the history of the commits that are being grafted onto the master branch, for example, use this command:

git filter-branch --tag-name-filter cat -- master..

Answer 2

Based on your diagrams (although I'm concerned about what you mean by "I'm pretty positive that I've fixed the issues causing git not to recognize that fact (using filter-branch)."), you should be able to do something like the following.

# checkout A
git checkout A

# Reset the branch pointer to E so that E is the parent of the next commit
# --soft ensures that the index stays the same
git reset --soft E

# Remake the commit with the E as the parent, re-using the old commit metadata
git commit -C HEAD@{1}

# Rebase the topic branch onto the modified A commit (current HEAD)
git rebase --onto HEAD A topic

Answer 3

All you need is this:

git rebase --root --onto master^^ topic^^ topic

the root option lets you include A.

UPDATE:

Add the --preserve-merges option if you want to retain the branching and merging of the part that you are rebasing.