splitting git commits

These days, 'git switch -c letstrythatagain' seems to be preferred, I think.

> cp bigfilewithlotsofchanges.py /tmp
> git reset --hard

I would do it differently. I'd first commit to a throw-away branch, so that I can then cherry-pick from it.

Also, most often, 'git add -p' (and then 's' for split) would allow committing the changes separately without needing that dance.

It's only when the changes are too close together that you'll need such a dance.
And when you need it, I'd do it as:

$ git add bigfilewithlotsofchanges.py
$ git commit -m "WIP; let's split this"
$ git tag tmp
$ git switch -c tryagain
$ git reset HEAD^ --h
## loop:
$ git checkout tmp -- bigfilewithlotsofchanges.py
$ git restore --staged .
$ vi bigfilewithlotsofchanges.py ## select the parts you want to keep for this commit
$ git add .
$ git commit -m "part 1"
## goto loop or finish.
## now we've finished, make sure that we're exactly like we started:
$ git diff tmp
$ git tag -d tmp

It's a long procedure, but that gives us a lot of control.