MutableRepo: record old checkout abandoned if it's empty

Same reasoning as the previous change.

With this change, I believe we now record all rewritten and abandoned
commits correctly. We're now almost ready to switch the CLI away from
using evolution for automatically rebasing commits.

lib/src/repo.rs

@@ -670,6 +670,7 @@ impl MutableRepo {
             CommitBuilder::for_rewrite_from(settings, self.store(), &current_checkout)
                 .set_pruned(true)
                 .write_to_repo(self);
+            self.record_abandoned_commit(current_checkout_id);
         }
         let store = self.store();
         // Create a new tree with any conflicts resolved.