Ref T13546. When we find commits in history which are associated with already-closed revisions, and they weren't named explicitly on the command line, and we're using a squash strategy, discard them.

This generally happens when "feature2" is on top of "feature1", but "feature1" gets amended or branched elsewhere and lands independently.

Ran "arc land feature3" where prior revisions had already landed, got discards on the duplicated changes.