Planner: don't use the planned dive for relative start time

The existing code has an embarrassing error in its logic. It picked the
last dive in the table and made sure that the relative start time was
either N minutes after 'now' or N minutes after the last dive ends,
whichever is later.

But once the planned dive has been added to the dive list (so once we have
a first depth and time entry, that last dive now is the planned dive. And
every time focus left the start time field the start time would be
recalculated relative to the end of the dive we are currently planning.

With this patch we instead simply remember the number of the last dive
just as we create the dive plan and use that to look up the end time of
previous dive. I could have just stored that end time but I figured maybe
there could be other reasons to go back to the last dive before the
planned dive, so this seemed cleaner.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>