git rebase. How Do I Use it to Collapse Reams of Ancient Commits

Question

I now have a big honking, bloated, Git repository consuming mucho disk space on GitHub that I want to put on a diet. I need to discard ancient commits made early in the history of the project that are are essentially irrelevant to the current direction the project is going.

I am - and will always be - the sole user of this private repo.

Ideally I could do something like:

git rebase from-birth-of-repo-until-one-month-ago

Thanks,
Doug

Answer 1

The --squash option to git merge might be useful, and is available in git 1.4.1 and later. This stages the effects of a merge, but doesn't create a commit. So, if 143eff is the oldest commit you want to include in the squashed commit, your current branch is master and the "one month ago" commit is dcb7e5, you could do:

# Save the old position of "master" by creating a branch old-master:
$ git checkout master
$ git branch old-master

# Create and checkout a branch called "new-master" that's at the old commit:
$ git checkout -b new-master 143eff

# Stage the effects of merging the "one month ago" commit:
$ git merge --squash dcb7e5
Updating 143eff3..dcb7e5b
Fast-forward
Squash commit -- not updating HEAD
[... status output showing the staged changes ..]

# Create the squashed commit:
$ git commit -m "A commit squashing history up to a month ago"
# (You could use --amend if you want them to be squashed into 143eff
# instead of being a commit after that.)

Now you can check with git diff dcb7e5 new-master that they really are the same.

Next, you want to rebase the rest of your work onto new-master:

$ git rebase --onto new-master dcb7e5 master

That will leave you on a rebased master which should have the history you want. (Again, you can check this with git diff old-master master and git log. When pushing master to github you will need to add --force since you've rewritten history:

# Push master to github, with "--force", since you've rewritten history:
$ git push --force origin master

You can now remove new-master, which is at the squash commit, with:

git branch -d new-master

Apparently github runs git gc --auto on pushes, so you should see some space saving soon...