Renumber windows: Lookup lastw via window not index
When calling 'movew -r' on a session to reorder the winlinks, ensure when
adding back in the information for the lastw stack that we look up the
winlink based on the window and not its index.
Using the index doesn't make sense here because when comparing it to the old
set, it will never match since the winlink has been renumbered.
Bug reported by Ben Boeckel.