Simplify entities for branches and tags API

## What does this MR do?

As talked in !5208, I improved the entity for branches and tags.

## Does this MR meet the acceptance criteria?

~~- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added~~
~~- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)~~
~~- [ ] API support added~~
~~- Tests~~
  ~~- [ ] Added for this feature/bug~~
  ~~- [ ] All builds are passing~~
~~- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)~~
~~- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)~~
~~- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)~~

See merge request !5343