multi-dive editing: don't change fields that weren't changed for the master dive

multi-dive editing: don't change fields that weren't changed for the master dive

Commit 2f773b97e042 ("multi-dive editing: don't change already set data
for other dives") didn't get the multi-dive editing quite right: even if
one of the dives in the list of changed dives has an empty field, we
should *not* fill it with the edit data unless that edit data was
actually changed.

So compare the new data with the original master data, and if they
match, do nothing.

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