The diff stats are no longer part of the diff generation.
Use `GetDiffShortStat` instead to get the total number of changed files,
added lines, and deleted lines.
As such, `gitdiff.GetDiff` can be simplified:
It should not do more than expected.
And do not run "git diff --shortstat" for pull list. Fix#31492
This module is merged from https://github.com/go-gitea/git which is a Go module to access Git through shell commands. Now it's a part of gitea's main repository for easier pull request.