unsaved changes on fork should go to the new notebook

[gordonwoodhull]

This is a helpful workflow when you realize your changes should go somewhere else.

1. open a notebook
2. start changing stuff but don't save
3. before autosave kicks in, fork the notebook

Expectation: old notebook is unchanged, fork has an extra commit with the new changes.

Instead, the changes are applied to the old notebook first, and the fork is identical.

This might actually be tricky to get right, since RCloud relies on saving everything on every change, but I wanted to file an issue anyway, since my expectation was broken here.