More multi-edit fixes

This time for values that aren't simply text.
For normal integers this is rather straight forward. For the 'when'
timestamp we simply assume that this is a shift in time.

What is still missing is consistent handling of the three fields that are
implemented as tags: tags, buddy and divemaster. We have special code for
tags that makes no sense in a multi-edit scenario. And we treat divemaster
and buddy as a single string - which kinda works but treats "Bill, Joe"
and "Joe, Bill" as different.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Simplify string comparison

This is based on Linus' idea on the mailing list.
Treat NULL strings and empty strings as identical.

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