cat: optimize rowskey hot loop

by precomputing group flag and bytes conversion outside loop

src/cmd/cat.rs

@@ -315,6 +315,9 @@ impl Args {
  GroupKind::None => {},
  }
 
+ let group_flag = group_kind != GroupKind::None;
+ let grouping_value_bytes = grouping_value.as_bytes();
+
  while rdr.read_byte_record(&mut row)? {
  new_row.clear();
  for (col_idx, c) in columns_global.iter().enumerate() {
@@ -324,10 +327,10 @@ impl Args {
  } else {
  new_row.push_field(b"");
  }
- } else if group_kind != GroupKind::None && col_idx == 0 {
+ } else if group_flag && col_idx == 0 {
  // we are in the first column, and --group is set
  // so we write the grouping value
- new_row.push_field(grouping_value.as_bytes());
+ new_row.push_field(grouping_value_bytes);
  } else {
  new_row.push_field(b"");
  }