Reduce padding on collapsed diff message

## What does this MR do?

Reduce the padding on the collapsed diff message so it takes up less space.

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

## Why was this MR needed?

Collapsed diffs took up too much space before.

## Screenshots (if relevant)

Before:

![image](/uploads/62c6fe19439bc3bc91bde83df72e7004/image.png)

After:

![image](/uploads/ec89230065c37fc4022d949d1233a90d/image.png)

## 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~~
  - [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)

See merge request !5256