Speed up methods that depend on diff stats only

What does this MR do?

Speeds up and reduces memory usage of Commit#repo_changesloop, Repository#expire_avatar_cache and IrkerWorker. It's a WIP because it need a new version of gitlab_git released with these changes.

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?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together