Validate pagination params in Branches API

Pavan Kumar Sadashivrequested to merge
gitlab-community/gitlab-org/gitlab:hrlpavan-601059-branches-api-verify-pagination-params into master

What does this MR do and why?

Calls verify_pagination_params! in API::Branches so that requests with zero or negative page or per_page values return a 400 Bad Request (page does not have a valid value / per_page does not have a valid value) when the only_positive_pagination_values feature flag is enabled, matching API::Commits.

References

How to set up and validate locally

bundle exec rspec spec/requests/api/branches_spec.rb

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