Use the divetrip index to look up divelist trip entries

We used to look up dive trips by their date, but these days we always
create a dynamic index for a dive trip when we insert it into the
divelist model, so we can use that to unambiguously match up dive trips
with the dive model entries.

That means that we don't get confused if we have two trips with the
exact same time, which happens when you load all the test-dives, for
example.

Reported-by: Miika Turkia <miika.turkia@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>