I ran `arc land --test` and verified the rebase used by `ArcanistMercurialLandEngine` was run with the flag to enable the extension.
```
$ hg --encoding utf-8 --config 'extensions.rebase=' rebase --dest 2fd68f4c48aaab9f6d5613b90e60b8219a9af0ae --rev 6132da63090c2c8a8d479aaae7b20497dda3e8fd..6132da63090c2c8a8d479aaae7b20497dda3e8fd --logfile - --keep --collapse
```
I modified the merging to throw an exception before writing to the future's stdin and verified that after `arc` completed there was no rebase active.