refreshRebaseCommits now computes the merged rebasing commits and working
tree state on the worker and writes Model.Commits /
WorkingTreeStateAtLastCommitRefresh in an onUIThreadUnlessRepoChanged
bounce. LocalCommitsMutex is left in place for now; it's shared with the
commits and branches refreshes and comes out once they're all bounced.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>