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/api/v1/repo/pull.go

@@ -1628,7 +1628,7 @@ func GetPullRequestFiles(ctx *context.APIContext) {
 	maxLines := setting.Git.MaxGitDiffLines
 
 	// FIXME: If there are too many files in the repo, may cause some unpredictable issues.
-	diff, err := gitdiff.GetDiff(ctx, baseGitRepo,
+	diff, _, err := gitdiff.GetDiffSimple(ctx, baseGitRepo,
 		&gitdiff.DiffOptions{
 			BeforeCommitID:     startCommitID,
 			AfterCommitID:      endCommitID,

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,

routers/web/repo/compare.go

@@ -621,7 +621,7 @@ func PrepareCompareDiff(
 
 	fileOnly := ctx.FormBool("file-only")
 
-	diff, err := gitdiff.GetDiff(ctx, ci.HeadGitRepo,
+	diff, err := gitdiff.GetDiffFull(ctx, ci.HeadGitRepo,
 		&gitdiff.DiffOptions{
 			BeforeCommitID:     beforeCommitID,
 			AfterCommitID:      headCommitID,

routers/web/repo/pull.go

@@ -981,7 +981,7 @@ func viewPullFiles(ctx *context.Context, specifiedStartCommit, specifiedEndCommi
 	// as the viewed information is designed to be loaded only on latest PR
 	// diff and if you're signed in.
 	if !ctx.IsSigned || willShowSpecifiedCommit || willShowSpecifiedCommitRange {
-		diff, err = gitdiff.GetDiff(ctx, gitRepo, diffOptions, files...)
+		diff, err = gitdiff.GetDiffFull(ctx, gitRepo, diffOptions, files...)
 		methodWithError = "GetDiff"
 	} else {
 		diff, err = gitdiff.SyncAndGetUserSpecificDiff(ctx, ctx.Doer.ID, pull, gitRepo, diffOptions, files...)