Returns correct header data for commits endpoint

app/models/repository.rb

...agination-headers-for-repository-commits-api-endpoint.yml

...sed/1381-present-commits-pagination-headers-correctly.yml

...elogs/unreleased/1381-use-commit-count-for-pagination.yml

doc/api/v3_to_v4.md

lib/api/commits.rb

lib/gitlab/git/repository.rb

spec/lib/gitlab/git/repository_spec.rb

spec/models/repository_spec.rb

spec/requests/api/commits_spec.rb