Sometimes discard already-closed revisions in "arc land"

Summary:
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.

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

Maniphest Tasks: T13546

Differential Revision: https://secure.phabricator.com/D21322