How to add a file to the first commit on a git repo?

Question

I have been converting old SVN repos to GIT. I have gotten the branches and tags to convert. Including making SVN tags real tags in GIT.

However, I would like to add a gitignore to the very first commit in the newly created GIT repo. I want it to be as if the file had always been there. So all the commits that followed (on master or in branches) would now have this file as the parent tree(s) would lead back the the first commit.

It seems that some form of git rebase or git filter-branch --tree-filter is what I need. I have tried each of these but I end up with disconnected branches.

Answer 1

In case anyone needs to do this in the future, here's an easier solution. I recently had to modify an early commit because of a problematic typo in the commit message. This works for changing anything about a previous commit. I modified Charles Bailey's answer here.

# Go back to the commit you want to change (detach HEAD)
git checkout <sha1_for_commit_to_change>

# Make any changes now (add your new file) then add them to the index
git add <new_files>

# amend the current commit
git commit --amend

# temporarily tag this new commit
# (or you could remember the new commit sha1 manually)
git tag tmp

# go back to the original branch (assume master for this example)
git checkout master

# Replay all the commits after the change onto the new initial commit
git rebase --preserve-merges --onto tmp <sha1_for_commit_to_change>

# remove the temporary tag
git tag -d tmp

Note that this will change all of your commit IDs following the amended commit, so it's not recommended on a public repository. Also, you'll have to recreate all of your tags

Answer 2

Do this on a clean working tree, on a new clone if you wish:

$ git checkout -b withgitignore $firstcommithash
$ git add .gitignore
$ git commit --amend
$ for branch in branch1 branch2 branch3 ... ; do
      git checkout $branch
      git rebase withgitignore
   done

Untested ;)