chore: simplify GetDiff (#7682)

- Split `GetDiff` into two functions, `GetDiffSimple` and `GetDiffFull`. The former will do the bare minimum and really only get a Git diff while the latter does some extra stuff that's relevant for the frontend to show extra relevant.
- Use `GetDiffSimple` for API related calls, as they do not benefit nor are returning the extra information that `GetDiffFull` provides, this should show a measurable performance increase for API calls that returns commits and `/repos/{owner}/{repo}/pulls/{index}/files`.
- `GetDiffSimple` contains extra code comments about its interesting way to determine the before commit.
- Add unit tests to demonstrates that the logic for determining the before commit didn't change and the function still yields correct information.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7682
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

services/convert/git_commit.go

@@ -210,7 +210,7 @@ func ToCommit(ctx context.Context, repo *repo_model.Repository, gitRepo *git.Rep
 
 	// Get diff stats for commit
 	if opts.Stat {
-		diff, err := gitdiff.GetDiff(ctx, gitRepo, &gitdiff.DiffOptions{
+		diff, _, err := gitdiff.GetDiffSimple(ctx, gitRepo, &gitdiff.DiffOptions{
 			AfterCommitID: commit.ID.String(),
 		})
 		if err != nil {