[libzstd] Optimize ZSTD_insertBt1() for repetitive data

[terrelln]

We would only skip at most 192 bytes at a time before this diff.
This was added to optimize long matches and skip the middle of the
match. However, it doesn't handle the case of repetitive data.

This patch keeps the optimization, but also handles repetitive data
by taking the max of the two return values.

Before:

> for n in $(seq 9); do echo strategy=$n; dd status=none if=/dev/zero bs=1024k count=1000 | command time -f %U zstd --zstd=strategy=$n >/dev/null; done
strategy=1
0.34
strategy=2
0.36
strategy=3
0.41
strategy=4
0.93
strategy=5
1.33
strategy=6
8.41
strategy=7
31.03
strategy=8
31.13
strategy=9
123.05

After:

> for n in $(seq 9); do echo strategy=$n; dd status=none if=/dev/zero bs=1024k count=1000 | command time -f %U ./zstd --zstd=strategy=$n >/dev/null; done
strategy=1
0.27
strategy=2
0.23
strategy=3
0.27
strategy=4
0.43
strategy=5
0.56
strategy=6
0.43
strategy=7
0.34
strategy=8
0.34
strategy=9
0.35

At level 19 with multithreading the compressed size of silesia.tar regresses 300 bytes, and enwik8 regresses 100 bytes. In single threaded mode enwik8 is also within 100 bytes, and I didn't test silesia.tar. The compression speed is not significantly effected.

The regression tests show changes in the order of single digit number of bytes.

Fixes Issue #1634.