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>

routers/web/repo/commit.go

@@ -329,7 +329,7 @@ func Diff(ctx *context.Context) {
 		maxLines, maxFiles = -1, -1
 	}
 
-	diff, err := gitdiff.GetDiff(ctx, gitRepo, &gitdiff.DiffOptions{
+	diff, err := gitdiff.GetDiffFull(ctx, gitRepo, &gitdiff.DiffOptions{
 		AfterCommitID:      commitID,
 		SkipTo:             ctx.FormString("skip-to"),
 		MaxLines:           maxLines,