Refs T13659, D21697.

I suppose I missed these cases.

• 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.

• I set up 3 dependent diffs and landed the middle revision and verified the first and second revisions landed but not the tip revision and there were no errors and the resulting working directory was correct.