Fix deprecation: You are passing an instance of ActiveRecord::Base to find

What does this MR do?

Fixes : DEPRECATION WARNING: You are passing an instance of ActiveRecord::Base to `find`. Please pass the id of the object by calling `.id`. (called from perform at /builds/gitlab-org/gitlab-ee/app/workers/rebase_worker.rb:7)

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Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.