interesting little bug...

[Dirk Hohndel]

Diving with multiple dive computers. For some reason (let's ignore why)
one of the computers has air as first gas and stage as second gas.
Second computer has air as second gas and stage as first.

Downloading from one then the other really gets us confused. We now have
both tanks as the deco gas. And with the identical gases of course gas
changes now aren't matched correctly, either...

So when we merge dives we need to actually match the gases instead of
just assuming gas 1 == gas 1.

Linus, this is your code - something you want to look at or should I
dive into it?

/D