Use at your own risk.
- Clone target repo with "git clone --bare https://github.com/user/other-repo.git"
- Clone this repo
- Copy authors.yml.example to target repo as authors.yml
- Edit authors.yml in target repo, with details of authors that need fixing
- Run fix-authors.sh inside target repo
- Hope it worked
$ git clone --bare https://github.com/user/other-repo.git
$ git clone https://github.com/jameshclrk/git-fix-authors.git
$ cd other-repo.git
$ cp ../git-fix-authors/authors.yml.example ./authors.yml
$ vim authors.yml
$ ../git-fix-authors/fix-authors.sh