Fix missing error check of bufio.Scanner (#29882)

maybe more

routers/web/repo/compare.go

@@ -980,5 +980,9 @@ func getExcerptLines(commit *git.Commit, filePath string, idxLeft, idxRight, chu
 		}
 		diffLines = append(diffLines, diffLine)
 	}
+	err = scanner.Err()
+	if err != nil {
+		return nil, fmt.Errorf("scan: %w", err)
+	}
 	return diffLines, nil
 }