Speed up methods that depend on diff stats only

## What does this MR do?
Speeds up and reduces memory usage of `Commit#repo_changes`loop, `Repository#expire_avatar_cache` and `IrkerWorker`.  It's a WIP because it need a new version of `gitlab_git` released with [these changes](https://gitlab.com/gitlab-org/gitlab_git/merge_requests/109).

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

## Why was this MR needed?
For large files committed, this method ends up loading them in memory which can consume lots of memory.

## What are the relevant issue numbers?
https://gitlab.com/gitlab-org/gitlab-ce/issues/19441#note_13425892

## Screenshots (if relevant)
N/A

## 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~~
  - [ ] ~~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 !5568