[PATCH] Speed up repeat() for larger counts

Hi all,

This patch optimizes repeat(text, int).

The current implementation constructs the result by copying the source text
once per repetition.  For larger repeat counts, that leads to many small
memcpy() calls.  This patch keeps the existing linear path for small
repeat counts, and uses a doubling strategy for larger ones by repeatedly
copying the already-generated prefix of the destination buffer.

In local testing, this reduced total runtime in a small repeat()-focused
microbenchmark, with the largest gains appearing in medium and large repeat
count cases.  Small-count cases did not show obvious regressions after
keeping the original path below a small cutoff.

The patch is attached.

Thanks for your time, and any feedback would be appreciated.

Best regards,
Chenhui Mo