undo/mobile: keep track of dive sites

When editing the dive site of a dive, the dive-table of the
corresponding dive site was not properly updated by the undo
commands. Try to get this right.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

commands/command_edit.h

@@ -386,6 +386,7 @@ public:
 private:
 	dive *oldDive; // Dive that is going to be overwritten
 	OwningDivePtr newDive; // New data
+	dive_site *newDiveSite;
 	int changedFields;
 
 	dive_site *siteToRemove;