[v12.0/forgejo] fix: correctly get stats for API commits (#8758)

**Backport:** https://codeberg.org/forgejo/forgejo/pulls/8756

- Instead of generating a patch and parsing its contents, use a faster and simple way to get it via `--shortstat`.
- Resolves forgejo/forgejo#8725
- Regression of forgejo/forgejo#7682
- Adds unit test.
- Adds integration test.

Co-authored-by: Gusted <postmaster@gusted.xyz>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8758
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: forgejo-backport-action <forgejo-backport-action@noreply.codeberg.org>
Co-committed-by: forgejo-backport-action <forgejo-backport-action@noreply.codeberg.org>

services/convert/git_commit.go

@@ -15,7 +15,6 @@ import (
 	api "forgejo.org/modules/structs"
 	"forgejo.org/modules/util"
 	ctx "forgejo.org/services/context"
-	"forgejo.org/services/gitdiff"
 )
 
 // ToCommitUser convert a git.Signature to an api.CommitUser
@@ -210,17 +209,15 @@ func ToCommit(ctx context.Context, repo *repo_model.Repository, gitRepo *git.Rep
 
 	// Get diff stats for commit
 	if opts.Stat {
-		diff, _, err := gitdiff.GetDiffSimple(ctx, gitRepo, &gitdiff.DiffOptions{
-			AfterCommitID: commit.ID.String(),
-		})
+		_, totalAdditions, totalDeletions, err := gitRepo.GetCommitShortStat(commit.ID.String())
 		if err != nil {
 			return nil, err
 		}
 
 		res.Stats = &api.CommitStats{
-			Total:     diff.TotalAddition + diff.TotalDeletion,
-			Additions: diff.TotalAddition,
-			Deletions: diff.TotalDeletion,
+			Total:     totalAdditions + totalDeletions,
+			Additions: totalAdditions,
+			Deletions: totalDeletions,
 		}
 	}