Include the last command state before the backstop cmds begin

The filtering on command states was using tstop instead of tstart
to avoid dealing with the last state ending in 2099.  Unfortunately,
this effectively discarded the last state.  This code change keeps
and appropriately truncates the last cmd state before the states
built from the backstop commands.
Merge pull request #81 from sot/use_last_prebs_state

Include the last command state before the backstop cmds begin