If you want to change the commit message for commit abcdef1 while preserving the commit history, you can use the git filter-branch command. Here are the steps to follow:

1. Create a backup of your repository before making any changes. This can be done by simply making a copy of the repository directory.
2. Run the following command to rewrite the commit message for abcdef1: 
```
git filter-branch -f --msg-filter 'sed "s/old message/new message/g"' abcdef1..HEAD
```
This command uses the sed command to replace the old commit message with the new one. Replace old message and new message with the actual commit messages.

3. Once the command finishes running, check the commit history by running git log. You should see that the commit message for abcdef1 has been updated, but the rest of the commit history remains intact.
4. Force push the changes to the remote repository by running git push --force. This is necessary because the commit history has been rewritten and the old commits are no longer compatible with the new history.
5. After force pushing the changes, any branches that depend on the old commit history will need to be updated. You can update the 12-fix-a and 13-fix-b branches by running the following command for each branch:
```
git checkout 12-fix-a
git merge origin/12-fix-a
```

This will update the branch to the latest commit on the remote repository and reapply any local changes.

Note that rewriting commit history can cause issues if other contributors have already pulled the changes from the remote repository. In this case, it may be better to simply add a new commit that updates the commit message and leave the original commit intact.
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