In WT-12598 I added more backup support to wtperf. At the time I made only a small change such that all backups actually taken were full backups (even if the backup introduced an incremental identifier). That allowed the tool to compare the content of files across backups.

POCs that change what/how data gets written to disk can be shown with that.

Now that some POCs are trying to change what gets copied taking an actual incremental backup is important (i.e. the data changed on disk in the database but is from an old checkpoint so that block doesn't need to be copied). In that case, copying the full table would show as changed when analyzing between backups.

This ticket fixes wtperf to actually take an incremental backup.