Reset diff count in git subtree
When doing the split the --rejoin option can be used, which seems to exist exactly to reuse the splits that have already happened.
--rejoin::
This option is only valid for the split command.
After splitting, merge the newly created synthetichistory back into your main project. That way, futuresplits can search only the part of history that hasbeen added since the most recent --rejoin.
From https://github.com/apenwarr/git-subtree/blob/master/git-subtree.txt