So far, the only situation where we called SetSelectionRangeAndMode was one
where the range could only get larger (in startInteractiveRebaseWithEdit, in
which case update-ref todos can be inserted by the rebase). However, in the last
commit we introduced a new call site where the range can get smaller, including
being reduced to a single item. Since this is indistinguishable from a single
selection, set the mode to none in this case; without this, hitting escape would
seemingly do nothing because it collapses the empty range selection.
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