Load branch dropdowns asynchronously for cherry pick and revert dialogs

## Why was this MR needed and what does it do?

Previously every commit page was loading every branch onto the page twice, once each for the Cherry Pick and Revert modal dialogs. Most of the time they're never used, so that's a bit of a waste.

This loads them asynchronously by modifying the existing ref switcher code to also work for the branch selectors. This is better for performance.

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

Make sure Cherry Picking and Reverting still work (from my tests they seem to be fine).

## What are the relevant issue numbers?

Fixes #20108.

## Screenshots (if relevant)

Before:

![Screen_Shot_2016-08-01_at_4.35.21_PM](/uploads/6b76ea656fd00df52ab56df5309976dc/Screen_Shot_2016-08-01_at_4.35.21_PM.png)

![Screen_Shot_2016-08-01_at_4.35.52_PM](/uploads/d2699a4bfc4ad1a4d414429e7c50e60f/Screen_Shot_2016-08-01_at_4.35.52_PM.png)

After:

![Screen_Shot_2016-08-01_at_4.32.49_PM](/uploads/5faa6494b10dd9ac5e8850763a0c4018/Screen_Shot_2016-08-01_at_4.32.49_PM.png)

![Screen_Shot_2016-08-01_at_4.32.56_PM](/uploads/87c490bb375244e7d72421fd55e13e3f/Screen_Shot_2016-08-01_at_4.32.56_PM.png)

![Screen_Shot_2016-08-01_at_4.33.02_PM](/uploads/59ae65c0afc39d8939a9f36431815854/Screen_Shot_2016-08-01_at_4.33.02_PM.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
  - [x] 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)

cc: @iamphill @jschatz1 

See merge request !5607