It reads the selected index and the commits and branches models to
decide where to move the fixup commit. Take those as parameters,
captured on the UI thread by the callers, so the function can run its
rebase on a worker without reading the model there. No behavior change;
the callers still run on the UI thread for now.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>