Only show branches for revert / cherry-pick

## What does this MR do?

Stop showing tags in the revert and cherry-pick select options. You can't change a tag anyway.

## Are there points in the code the reviewer needs to double check?

Uncertain.

## Why was this MR needed?

Showing tags doesn't make any sense and will just throw an exception if the user tries to cherry-pick or revert onto a tag.

## What are the relevant issue numbers?

Fixes #18377.

## Screenshots (if relevant)

On the GitLab CE repo:

![image](/uploads/9c5a6f09300a7c46d0a794bef2956992/image.png)

## Does this MR meet the acceptance criteria?

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

See merge request !4596