handleReword, amendTo, and the reset/set/add-co-author handlers pass
Model().Commits (and the selected line index) to a git rebase from
inside the WithWaitingStatus worker, racing the UI thread's model
writes. Read them on the UI thread before dispatching.
The author handlers index the full commit list by absolute start/end, so
the range sub-slice withItemsRange hands amendAttribute is not what they
need; capture the full Model().Commits there and thread it through.
b4a976834f