Do a dive de-dup pass

Do a dive de-dup pass

If given multiple dives at the same time, just de-dup the dives.  This
happens when you've dumped the whole dive-computer several times, and
some dives show up in multiple dumps.

When de-duping, try to avoid dropping data.  So if one dive has notes
attached to it, and the other one does not, pick the notes from the dive
that does have them.  Obvious stuff like that.

The sample merge is also written so that it should be possible to merge
two dives. Which we don't actually do yet.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>