Currently editing of planned dives that have been merged with actual
(logged) dives only works if the 'Planned dive' divecomputer is the
first divecomputer, and this divecomputer is selected when clicking
'Edit planned dive'. In other cases the profile of the first
divecomputer is overlaid with the profile of the planned dive, and the
first divecomputer's profile is overwritten when saving the dive plan.
Fix this problem.
Triggered by @SeppoTakalo's comment (https://github.com/subsurface/subsurface/issues/1913#issuecomment-2075562119): Users don't like that planned dives show up as their own entries in the dive list, so being able to merge them with the actual dive after it has been executed is a good feature - but this wasn't working well until now.
Signed-off-by: Michael Keller <github@ike.ch>
