Enable nilnil linter for new code (#36591)

Fixes: https://github.com/go-gitea/gitea/issues/36152

Enable the `nilnil` linter while adding `//nolint` comments to existing
violations. This will ensure no new issues enter the code base while we
can fix existing issues gradually.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

routers/common/compare.go

@@ -161,7 +161,7 @@ func GetHeadOwnerAndRepo(ctx context.Context, baseRepo *repo_model.Repository, c
 
 func findHeadRepoFromRootBase(ctx context.Context, baseRepo *repo_model.Repository, headUserID int64, traverseLevel int) (*repo_model.Repository, error) {
 	if traverseLevel == 0 {
-		return nil, nil
+		return nil, nil //nolint:nilnil // return nil to indicate that the object does not exist
 	}
 	// test if we are lucky
 	repo, err := repo_model.GetUserFork(ctx, baseRepo.ID, headUserID)
@@ -185,5 +185,5 @@ func findHeadRepoFromRootBase(ctx context.Context, baseRepo *repo_model.Reposito
 			return forked, nil
 		}
 	}
-	return nil, nil
+	return nil, nil //nolint:nilnil // return nil to indicate that the object does not exist
 }