Fix logs for custom retention policy to show backup times.
Fix logs for custom retention policy to show backup times.

When we modified the code in revision
68ffd865947c443ec5d02ae4aa885a479e2f4ebd (pull request #4018) to
consider collections of IListResultFilesets instead of DateTimes, we
neglected to preserve the backup times in the logs.

This fixes #4109.

mnaiman/duplicati,mnaiman/duplicati,duplicati/duplicati,duplicati/duplicati,duplicati/duplicati,mnaiman/duplicati,mnaiman/duplicati,duplicati/duplicati,duplicati/duplicati,mnaiman/duplicati