Don't show gaschange events at the beginning of a dive

The initial gas change event is really special - it just specifies the gas
mix from the dive computer. So don't show it as an event if that already
matches the initial gas.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>