Refactor the existing functions to get shortstat of commit and between two commits to use performant alternatives, mainly `git-diff-tree` and `git-diff-index`.
Resolvesforgejo/forgejo#9551
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/9587
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>