Misleading "Ignoring incremental Backupset" message

[Tobias G. Pfeiffer]

For a while I have been doing my duplicity backups to Glacier now. First I used a kind of hack to do a normal AWS upload, then rename all the difftar files to match S3 lifecycle rules. Using that method, I got "Ignoring incremental Backupset" messages on subsequent backups, maybe because there were no data files found using the expected file name pattern.

Then recently I upgraded to the latest duplicity version and switched to the `--file-prefix-archive` method, which feels a lot better. However, I still had the "Ignoring incremental Backupset" message in my logs which confused me a bit, since all commands seem to behave correctly. I found out that when an incremental backupset is found, `chain.add_inc(set)` is called on all chains until one of these calls returns True, and every call before will emit an "Ignoring incremental Backupset" message. So the very normal operation of "finding the correct chain" emits a message that indicates something is ignored or missing or not matching etc.

I think the functionality of finding the correct chain should be changed so that no such warning message is emitted in order to avoid user confusion.