split: do not prevent all changes from going into the parent commit, and a fix

[samueltardieu]

Allowing to put everything into the first commit closes #4533.

Question: should we suggest using jj undo if everything was put into one of the two commits in case the user didn't want to do this? Or, do we want this only if, in addition, the empty commit has an empty description, because there should be no reason to create an empty commit?

Checklist

If applicable:

• [X] I have updated CHANGELOG.md
• [ ] I have updated the documentation (README.md, docs/, demos/)
• [ ] I have updated the config schema (cli/src/config-schema.json)
• [ ] I have added tests to cover my changes

[martinvonz]

Should most of the PR description be moved into the commit description (with s/PR/commit/ or similar)? The question makes sense in the PR description.

[samueltardieu]

Should most of the PR description be moved into the commit description (with s/PR/commit/ or similar)? The question makes sense in the PR description.

It even led me to split the commits (using jj split, this is appropriate).