Error 500 loading merge requests via API due to missing head_commit_sha for diff

https://sentry.gitlap.com/gitlab/gitlabcom/issues/475854/

NoMethodError: undefined method `head_commit_sha' for nil:NilClass
  merge_request.rb:441:in `diff_head_sha'
    merge_request_diff.head_commit_sha
  grape_entity/delegator/plain_object.rb:8:in `delegate'
    object.send attribute
  grape_entity/entity.rb:512:in `delegate_attribute'
    delegator.delegate(attribute)
  grape_entity/exposure/delegator_exposure.rb:8:in `value'
    entity.delegate_attribute(attribute)
  grape_entity/exposure/base.rb:82:in `valid_value'
    value(entity, options) if valid?(entity)
...
(137 additional frame(s) were not displayed)

NoMethodError: undefined method `head_commit_sha' for nil:NilClass