[SPARK-53171][CORE] Improvement UTF8String repeat

[yaooqinn]

What changes were proposed in this pull request?

This PR improves the UTF8String repeat logic for some special cases

Why are the changes needed?

performance improvement

Does this PR introduce any user-facing change?

now

How was this patch tested?

Passing existing UT and benchmarked like

scala> spark.time((1 to 10000000).foreach(_ => UTF8String.fromStri
ng("A").repeat(1024)))
Time taken: 784 ms
scala> spark.time((1 to 10000000).foreach(_ => repeat(UTF8String.f
romString("A"), 1024)))
Time taken: 432 ms

Was this patch authored or co-authored using generative AI tooling?

no

[dongjoon-hyun]

+1, LGTM. Nice!

Thank you, @yaooqinn .

[dongjoon-hyun]

Merged to master for Apache Spark 4.1.0.

[yaooqinn]

Thank you @dongjoon-hyun