RFR: 8336741: Optimize LocalTime.toString with StringBuilder.repeat

[Shaojin Wen]

class LocalTime {
    public String toString() {
        // ...
                if (nanoValue % 1000_000 == 0) {
                    buf.append(Integer.toString((nanoValue / 1000_000) + 1000).substring(1));
                } else if (nanoValue % 1000 == 0) {
                    buf.append(Integer.toString((nanoValue / 1000) + 1000_000).substring(1));
                } else {
                    buf.append(Integer.toString((nanoValue) + 1000_000_000).substring(1));
                }
       // ...
    }
}

Currently, LocalTime.toString handles nanos by adding a value and then subString(1) to fill it with zeros. Using StringBuilder.repeat is more concise and has better performance.

-------------

Commit messages:
 - use StringBuilder#repeat append zeros

Changes: https://git.openjdk.org/jdk/pull/20232/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20232&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336741
  Stats: 14 lines in 1 file changed: 10 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/20232.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20232/head:pull/20232

PR: https://git.openjdk.org/jdk/pull/20232