MergeWorker job may be run before squash parameter is saved

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This came from #32185 (comment 220152835). Right now it's possible that the merge flag isn't committed to the database before MergeWorker runs:

https://gitlab.com/gitlab-org/gitlab/blob/6f1bf83acdb68dc7eb2d83ec59c53ed5069b6a8e/app/controllers/projects/merge_requests_controller.rb#L274-287 updates the squash column.

We could fix this by:

1. Use run_after_commit if the squash parameter is updated
2. We appear to pass the squash parameter in MergeWorker. We could use that value as well.