This fixes a subtle bug introduced in 5c4569247a which
unified two functions finding the gasmix at a given time
during the dive. There was a slight difference, though:
Does a gaschange exactly at that time count or not? For
the planner to work, the answer has to be in the affirmative.
Signed-off-by: Robert C. Helling <helling@atdotde.de>
