Fix the diff in the merge request view when converting a symlink to a regular file

## What does this MR do?

It fixes a bug described in #21610

## Why was this MR needed?

There was a regression introduced in https://gitlab.com/gitlab-org/gitlab-ce/commit/cd7c2cb6ddd4d9c9f9bdae00c887c0022c121c17

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [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 it does - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

Fixes #21610

See merge request !6828