Split an old commit in git using vimdiff or meld.
Create a backup copy of your work.
Run git split at the root of your repository.
$ git split <commitish> <filename>git-split displays the previous contents of the file on the tab on left (.begin) and the future contents on the right (.end)
Edit the file in the central column and save it.
git-split then asks to commit the changes between the left and center column.
Later at asks to commit the changes between the center column and the right column.
To split a commit into more than two run git-split multiple times.