multi-dive editing: don't change already set data for other dives

When editing multiple dives at the same time, don't change fields that
have already been set for a dive, unless the old field contents match
the currently selected ("master") dive.

So when you edit multiple dives, you can set the dive master or buddy
(or suit etc) for all of them in one go, but if one of them already has
that field set, it won't be modified just because you set the other
ones.

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