Remove / cut off Git's revision / commit history

Question

I have a project that contains traces to another project of myself that I've used as a kind of a template project. Now I want to remove these traces entirely from the repository. Basically I want to cut off the old junk commits. So I have

A -- B -- C -- D -- E -- F

and want to get something like

D -- E -- F

with A -- B -- C being completely removed from the repository.

Answer 1

Assuming master is at commit F:

 # create a new branch with D's content
$ git checkout --orphan temp <d-sha1>
$ git commit

 # rebase everything else onto the temp branch
$ git rebase --onto temp <d-sha1> master

 # clean up
$ git checkout master
$ git branch -d temp

---

If you want to completely remove the old loose objects (A, B, & C), first make sure you have exactly what you want. This cannot be undone. Once you have confirmed it's what you want, run:

$ git reflog expire --expire=now --all
$ git gc --prune=now

Answer 2

Github has a good article about removing sensitive data (so the commits you want too):

Remove Sensitive Data from Git