Stop libdivecomputer import when we start seeing old dives

Stop libdivecomputer import when we start seeing old dives

I don't know about other dive computers, but the Suunto Vyper Air is
slow as hell to import all the dives from.  And libdivecomputer seems to
be importing dives "most recent first", so this just makes it stop
importing dives when it finds a dive that we've already seen.

Caveat: libdivecomputer has this fancy notion of "dive fingerprints",
and claims that's the way to do things.  That seems to be overly
complicated, and not worth the bother.

If you worry about the import finishing early due to already having some
dives with the same date in your dive list, just import starting from an
empty state, and thus get a pure "dive computer only" state with no
early out.  Then you can just load the old dives afterwards, and depend
on subsurface merging any duplicates.

But for normal operation, when you just want to import a couple of new
dives from your dive computer, the "exit import early when you see a
duplicate" is the right thing to do.

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