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..